A Peek Beyond the Point Class 7 Notes Maths Chapter 3

Students often refer to Class 7 Maths Notes and Chapter 3 A Peek Beyond the Point Class 7 Notes during last-minute revisions.

Class 7 Maths Chapter 3 Notes A Peek Beyond the Point

Class 7 Maths Notes Chapter 3 – Class 7 A Peek Beyond the Point Notes

→ We know that numbers help us measure and compare things in real life. In the previous class, we learnt about fractions and basic measurements, and now we take a step further by easily handling smaller parts, like tenths and hundredths. It helps in precise measurements, weight conversions, and even money calculations such as rupees and paise. Understanding decimals allows us to add, subtract, and compare them, solving everyday problems with accuracy and confidence!

→ When exact measures are required. we can make use of smaller units of measurement.

→ In the Indian place value system. the decimal point (.) is used to separate the whole number part of a number from its fractional part.

→ We extended the Indian place value system after the decimal point and saw that:

• 1 unit = 10 one-tenths
• 1 tenth = 10 one-hundredths
• 1 hundredth = 10 one-thousandths
• 10 one-hundredths = 1 tenth

→ Units of measurement: .

• 1 cm = 10 mm
• 1 cm = \(\frac{1}{100}\) m

→ Weight Conversion: 1 g = \(\frac{1}{1000}\) kg
→ Rupee-Paise conversion: 1 rupee = 100 paise

→ We can split a unit into smaller parts to get more exact/accurate measurements.

→ We extended the Indian place value system and saw that

• 1 unit = 10 one-tenths
• 1 tenth = 10 one-hundredths
• 1 hundredth = 10 one-thousandths
• 10 one-hundredths = 1 tenth
• 100 one-hundredths = 1 unit

→ A decimal point (‘.’) is used in the Indian place value system to separate the whole number part of a number from its fractional part.

→We also learnt how to compare decimal numbers, locate them on the number line, and perform addition and subtraction.

Smaller Units

→ Smaller units of measurements are used where the exact measures are required.

A Tenth Part

→ A tenth part is one of ten equal parts of something.

→ It is expressed as \(\frac{1}{10}\).

→ e.g. If Ram has a packet of 10 biscuits and he distributes one biscuit to each of his 10 friends. Then, a biscuit is a tenth part of the whole packet.
Note 10 one-tenths units = 1 unit

→ Fractional units along with how to read them are
e.g. 4 \(\frac{1}{10}\) → ‘four and one-tenth’
\(\frac{4}{10}\) → ‘four one-tenths’ or ‘four-tenths’

A Hundredth Part

→ A hundredth part is one of hundred equal parts of something.
It is expressed as \(\frac{1}{100}\).

→ e.g. Ramesh buys a packet of rice. He divides the rice into 100 equal parts and distributes among 100 poor people. Then each poor person gets one-hundredth part of the whole packet.
Note 100 one-hundredths units = 1 unit

→ 10 one-hundredths units = one-tenth unit

→ Some fractional units along with how to read them are
e.g. 1 \(\frac{1}{10}\) \(\frac{4}{100}\)

→ One and one-tenth and four-hundredths
1 \(\frac{14}{100}\) → One and fourteen-hundredths
\(\frac{114}{100}\) → One hundred and fourteen-hundredths

Decimal Place Value

→ A decimal point (.) is used in the Indian place system to separate the whole number part of a number from its fraction part.

→ In decimal notation, \(\frac{1}{10}\) = 0.1 and read as ‘zero point one’. The ‘dot’ represents the decimal point.

→ \(\frac{1}{100}\) = 0.01 and read as ‘zero point zero one’.

→ \(\frac{1}{1000}\) = 0.001 and read as ‘zero point zero zero one’.

Fractions as Decimals

→ Every fraction with denominator 10 or 100 can be written in decimal notation.

→ So, for writing the fraction in decimal notation upto tenths or hundredths place, we make denominator 10 or 100 by multiplying numerator and denominator of fraction with a suitable number.

→ e.g.\(\frac{11}{5}\) = \(\frac{11 \times 2}{5 \times 2}\) = \(\frac{22}{10}\) = 2.2

Decimals as Fractions

→ Firstly we write the given decimal into fraction with denominator 10 or 100 and then simplify to write in lowest form.
e.g. 1.2 = \(\frac{12}{10}\) = \(\frac{6}{5}\)
[dividing numerator and denominator by 2 ]

Locating Decimals

→ All decimals can also be represented on a number line. Draw a number line and divide the unit length between two whole numbers into 10 equal parts, where each of these equal parts represents 0.1 or \(\frac{1}{10}\).

→ Now, we locate the given decimals on this line as

e.g. 0.4 on number line as

→ For locating one-hundredths part, we divide the number line between two tenths into 10 equal parts, where each of these equal parts represents 0.01 or \(\frac{1}{100}\).

→ Now, we locate this as

e.g. 0.45 on number line as

Comparing Decimals

→ For comparing decimals, we first convert them into like decimals and then compare the whole part, where the decimal number having greater whole part will be greater.

• If the whole part is same for both, then compare the tenths part and decimal having greater tenths part will be greater.
• If tenths part is also same, then compare the hundredths part and decimal having greater hundredths part will be greater.
• If hundredths part is same, then compare the thousandths part and find greater decimal number and so on.

→ e.g. In 1.09 and 1.093, converting them into like decimals, we get 1.090 and 1.093. We find that the whole part, tenths part, hundredths part are same and thousandths part of 1.093 and 1.090 are not i.e. 3 > 0.
So, 1.093 > 1.09.

→ Note (i) Like decimals have same number of decimal point.
(ii) Without changing the value of a decimal, the number of decimal places can be increased simply by adding zeroes to the extreme right of its decimal parts.

Addition and Subtraction of Decimals

Addition of Decimals

→ To add the numbers with decimals, we follow the steps given below

• Convert the given decimals into like decimals.
• Write the numbers according to the place value chart or arrange in columns in such a way that the digits of same place are arranged in same columns.
• Now, add the numbers in column from the right, the same way as we carry the regular addition.

Subtraction of Decimals

→ To subtract the decimal number, we follow the steps given below

• Convert the given decimals into like decimals.
• Write the smaller number below the larger one in column form in such a way that the decimal points of both the numbers are in the same column and the digits of the same place are arranged in same column.
• Subtract as we do in case of whole numbers.
• Put the decimal point directly under the decimal points of the given numbers.

→ Notlae sum will be greater than the sum of their whole parts and less than the sum of whole parts plus two.
The difference will be greater than the difference of their whole parts and less than the difference of whole parts plus one.

Closest Decimal

→ The closest decimal means finding a decimal number that is nearest to a given number. It is usually done by rounding the decimal to a specific place value like-tenths, hundreadths or thousandths.

Decimal Sequences

→ A decimal sequence is a list of decimal numbers arranged in a specific order, usually in increasing or decreasing order.

Units of Measurement

→ Length Conversion

• 1 cm (centimetre) = 10 mm (millimeters)
  i.e. 1 mm = \(\frac{1}{10}\) cm (i.e. one-tenth of a centimetre)
• 1 m (metre) = 100 cm (centimetres)
  i.e. 1 cm = \(\frac{1}{100}\) m (i.e. one-hundredth of a metre)
• 1 km (kilometre) = 1000 m (metres)
  i.e. 1 m = \(\frac{1}{1000}\) km

The Need for Smaller Units Class 7 Notes

Sonu’s mother was fixing a toy. She was trying to join two pieces with the help of a screw. Sonu was watching his mother with great curiosity. His mother was unable to enter the pieces. Sonu asked why. His mother said that the screw was not of the right size.

She brought another screw from the box and was able to fix the toy. The two screws looked the same to Sonu. But when he observed them closely, he saw they were of slightly different lengths.

Sonu was fascinated by how such a small difference in lengths could matter so much. He was curious to know the difference in lengths. He was also curious to know how little the diffrence was because the screws looked nearly the same.

In the following fiure, screws are placed above a scale. Measure them and write their length in the space provided.

What is the meaning of 2\(\frac{7}{10}\) cm (the length of the fist screw)?
As seen on the ruler, the unit length between two consecutive numbers is divided into 10 equal parts.
To get the length 2\(\frac{7}{10}\) cm, we go from 0 to 2 and then take seven parts of \(\frac{1}{10}\).
The length of the screw is 2 cm and \(\frac{7}{10}\) cm.
Similarly, we can make sense of the length 3\(\frac{2}{10}\) cm.
We read 2\(\frac{7}{10}\) cm as two and seven-tenth centimeters, and 3\(\frac{2}{10}\) cm as three and two-tenth centimeters.

Write the measurements of the objects shown in the picture:

As seen here, when exact measures are required, we can make use of smaller units of measurement.

A Tenth Part Class 7 Notes

The length of the pencil shown in the figure below is 3\(\frac{4}{10}\) units, which can also be read as 3 units and four one-tenths, i.e., (3 × 1) + (4 × \(\frac{1}{10}\)) units.

This length is the same as 34 one-tenths unit because 10 one-tenths unit makes one unit.
\(34 \times \frac{1}{10}=\frac{34}{10}=\frac{10}{10}+\frac{10}{10}+\frac{10}{10}+\frac{4}{10}\) (34 one-tenths)
= 1 + 1 + 1 + \(\frac{4}{10}\) (3 and 4 one-tenths)
A few numbers with fractional units are shown below, along with how to read them.
4\(\frac{1}{10}\) → ‘four and one-tenth’
\(\frac{4}{10}\) → ‘four one-tenths’ or ‘four-tenths’
\(\frac{41}{10}\) → ‘forty-one one-tenths’ or ‘forty-one tenths’
41\(\frac{1}{10}\) → ‘forty-one and one-tenth’

For the objects shown below, write their lengths in two ways and read them aloud. An example is given for the USB cable. (Note that the unit length used in each diagram is not the same.)
The length of the USB cable is 4 and \(\frac{8}{10}\) units or \(\frac{48}{10}\) units.

Sonu is measuring some of his body parts. The length of Sonu’s lower arm is 2\(\frac{7}{10}\) units, and that of his upper arm is 3\(\frac{6}{10}\) units. What is the total length of his arm?
To get the total length, let us see the lower and upper arm lengths as 2 units and 7 one-tenths, and 3 units and 6 one-tenths, respectively.
So, there are (2 + 3) units and (7 + 6) one-tenths. Together, they make 5 units and 13 one-tenths. But 13 one-tenths is 1 unit and 3 one-tenths.
So, the total length is 6 units and 3 one-tenths.

Or, both lengths can be converted to tenths and then added:
(c) 27 one-tenths and 35 one-tenths is 62 one-tenths
\(\frac{27}{10}+\frac{35}{10}=\frac{62}{10}\)
\(\frac{62}{10}\) is the same as 60 one-tenths (\(\frac{60}{10}\)) and 2 one-tenths (\(\frac{2}{10}\)), which is equal to 6 units and 2 one-tenths, i.e., 6\(\frac{2}{10}\).

The lengths of the body parts of a honeybee are given. Find its total length.

Head: 2\(\frac{3}{10}\) units
Thorax: 5\(\frac{4}{10}\) units
Abdomen: 7\(\frac{5}{10}\) units

The length of Shylaja’s hand is 12\(\frac{4}{10}\) units, and her palm is 6\(\frac{7}{10}\) units, as shown in the picture. What is the length of the longest (middle) finger?

The length of the finger can be found by evaluating (12 + \(\frac{4}{10}\)) – (6 + \(\frac{7}{10}\)). This can be done in different ways. For example,

As in the case of counting numbers, it is convenient to start subtraction from the tenths. We cannot remove 7 one-tenths from 4 one-tenths. So we split a unit from 12 and convert it to 10 one-tenths. Now, the number has 11 units and 14 one-tenths. We subtract 7 one-tenths from 14 one-tenths and then subtract 6 units from 11 units.

Smaller units are needed for precise measurements, especially when dealing with objects that are small or require accuracy.

Less accurate

More accurate. Each cm is divided into 10 equal parts. Its length is 2 cm and \(\frac{3}{10}\) parts of a centimeter

A Tenth Part
When a whole is divided into ten equal parts, each part is called one-tenth. It is written as \(\frac{1}{10}\).

In the above example, the cm was divided into 10 equal parts. Each part is called a millimeter (mm). Thus, the screw is 2 cm 3 mm long.

The length of the pencil shown in the figure below is 3\(\frac{7}{10}\) units, which can also be read as
3units and seventh one-tenths, i.e., (3 × 1) + (7 × \(\frac{1}{10}\)) units.

This length is the same as 37 one-tenths units because 10 one-tenths units make one unit.
Hence, 3\(\frac{7}{10}=\frac{37}{10}\) .

Addition and Subtraction
2\(\frac{3}{10}\) + 1\(\frac{4}{10}\) = 2 + \(\frac{3}{10}\) + 1 + \(\frac{4}{10}\)
= 2 + 1 + \(\frac{3}{10}+\frac{4}{10}\)
= 3 + \(\frac{7}{10}\)
= 3\(\frac{7}{10}\)

4\(\frac{7}{10}\) – 3\(\frac{2}{10}\) = 4 + \(\frac{7}{10}\) – 3 – \(\frac{2}{10}\)
= 4 – 3 + \(\frac{7}{10}-\frac{2}{10}\)
= 1 + \(\frac{5}{10}\)
= 1\(\frac{5}{10}\)

6\(\frac{5}{10}\) – 2\(\frac{7}{10}\) = 6 + \(\frac{5}{10}\) – 2 – \(\frac{7}{10}\)
= 5 + \(\frac{15}{10}\) – 2 – \(\frac{7}{10}\)
= 5 – 2 + \(\frac{15}{10}-\frac{7}{10}\)
= 3 + \(\frac{8}{10}\)
= 3\(\frac{8}{10}\)

A Hundredth Part Class 7 Notes

The length of a sheet of paper was 8\(\frac{9}{10}\) units, which can also be said as 8 units and 9 one-tenths. It is folded in half along its length. What is its length now?

We can say that its length is between 4\(\frac{4}{10}\) units and 4\(\frac{5}{10}\) units. But we cannot state its exact measurement, since there are no markings. Earlier, we split a unit into 10 one-tenths to measure smaller lengths. We can do something similar and split each one-tenth into 10 parts.

What is the length of this smaller part? How many such smaller parts make a unit length?
As shown in the figure below, each one-tenth has 10 smaller parts, and there are 10 one-tenths in a unit; therefore, there will be 100 smaller parts in a unit. Therefore, one part’s length will be \(\frac{1}{100}\) of a unit.

Returning to our question, what is the length of the folded paper?
We can see that it ends at \(4 \frac{4}{10} \frac{5}{100}\), read as 4 units and 4 one-tenths and 5 one-hundredths.

Observe the figure below. Notice the markings and the corresponding lengths written in the boxes when measured from 0. Fill in the lengths in the empty boxes.

The length of the wire in the first picture is given in three different ways. Can you see how they denote the same length?

\(1 \frac{1}{10} \frac{4}{100}\) → One and one-tenth and four-hundredths
1\(\frac{14}{100}\) → One and fourteen-hundredths
\(\frac{114}{100}\) → One Hundred and Fourteen-hundredths

For the lengths shown below, write the measurements and read out the measures in words.

In each group, identify the longest and the shortest lengths. Mark each length on the scale.

What will be the sum of \(15 \frac{3}{10} \frac{4}{100}\) and \(2 \frac{6}{10} \frac{8}{100}\)?
This can be solved in different ways. Some are shown below.
(a) Method 1

(b) Method 2

Observe the addition done below for 483 + 268. Do you see any similarities between the methods shown above?
(400 + 80 + 3) + (200 + 60 + 8)
= (400 + 200) + (80 + 60) + (3 + 8)
= 600 + 140 + 11
= 600 + 150 + 1
= 700 + 50 + 1
= 751
One can also find the sum \(15 \frac{3}{10} \frac{4}{100}+2 \frac{6}{10} \frac{8}{100}\) by converting to hundredths, as follows.

What is the diffrence: 25\(\frac{9}{10}\) – \(6 \frac{4}{10} \frac{7}{100}\)?
One way to solve this is as follows:

Solve this by converting to hundredths.
What is the diffrence \(15 \frac{3}{10} \frac{4}{100}-2 \frac{6}{10} \frac{8}{100}\)?
One way to solve this is as follows:

Observe the subtraction done below for 653 – 268. Do you see any similarities with the methods shown above?
(600 + 50 + 3) – (200 + 60 + 8)
= (600 – 200) + (50 – 60) + (3 – 8)
= (600 – 200) + (40 – 60) + (13 – 8)
= (600 – 200) + (40 – 60) + 5
= (500 – 200) + (140 – 60) + 5
= 300 + 80 + 5
= 385

When a whole is divided into hundred equal parts, each part is called one-hundredth. It is written as \(\frac{1}{100}\).
The length of the wire in the picture is given in three different ways:

1\(\frac{1}{2}\) → one and one-tenth and two-hundredths
1\(\frac{12}{100}\) → one and twelve-hundredths
\(\frac{112}{100}\) → one hundred and twelve-hundredths
A meter is divided into 100 equal parts. Each part is called a centimeter (cm).
Hence 1 cm is \(\frac{1}{100}\) m or one-hundredths of a meter.

Addition

Subtraction

A Thousandth Part
When a whole is divided into thousand equal parts, each part is called one-thousandth. It is written as \(\frac{1}{1000}\).
Example: 1\(\frac{2}{10} \frac{3}{100} \frac{7}{1000}\) → 1 one and 2 tenths 3 hundredths 7 thousandths
1\(\frac{237}{1000}\) → one and 237 thousandths
\(\frac{1237}{1000}\) → 1237 thousandths

Decimal Place Value Class 7 Notes

You may have noticed that whenever we need to measure something more accurately, we split a part into 10 (smaller) equal parts ― we split a unit into 10 one-tenths and then split each one-tenth into 10, and then we use these smaller parts to measure.

Can we not split a unit into 4 equal parts, 5 equal parts, 8 equal parts, or any other number of equal parts instead?
Yes, we can. The example below compares how the same length is represented when the unit is split into 10 equal parts and when the unit is split into 4 equal parts.

If an even more precise measure is needed, each quarter can be further split into four equal parts. Each part then measures \(\frac{1}{16}\) of a unit, i.e., 16 such parts make 1 unit.

Then why split a unit into 10 parts every time?
The reason is the special role that 10 plays in the Indian place value system. For a whole number written in the Indian place value system — for example, 281 — the place value of 2 is hundreds (100), that of 8 is tens (10), and that of 4 is ones (1). Each place value is 10 times bigger than the one immediately to its right. Equivalently, each place value is 10 times smaller than the one immediately to its left:
10 ones make 1 ten,
10 tens make 1 hundred,
10 hundreds make 1 thousand, and so on.

To extend this system of writing numbers to quantities smaller than one, we divide one into 10 equal parts. What does this give? It gives one-tenth. Further dividing it into 10 parts gives one-hundredth, and so on.

Can we extend this further? What will the fraction be when \(\frac{1}{100}\) is split into 10 equal parts?
It will be \(\frac{1}{1000}\), i.e., a thousand such parts make up a unit.

Just as when we extend to the left of 10,000, we get bigger place values at each step, we can also extend to the right of \(\frac{1}{1000}\), getting smaller place values at each step.

This way of writing numbers is called the “decimal system” since it is based on the number 10; “decem” means ten in Latin, which in turn is cognate to the Sanskrit daśha meaning 10, with similar words for 10 occurring across many Indian languages including Odia, Konkani, Marathi, Gujarati, Hindi, Kashmiri, Bodo, and Assamese. We shall learn about other ways of writing numbers in later grades.

• 2 and 3 tenths can be written as 2.3.
• 12 and 5 tenths can be written as 12.5.
• 47 tenths = 40 tenths and 7 tenths = 4 and 7 tenths = 4.7.
• 6 and 25 hundredths can be written as 6.25.
• 43 and 56 hundredths can be written as 43.56.
• 238 hundredths = 200 hundredths and 38 hundredths = 2 and 38 hundredths = 2.38.
• 2 × 1 + 3 × \(\frac{1}{10}\) = 2.3
• 7 × 10 + 5 × 1 + 4 × \(\frac{1}{10}\) = 75.4
• 3 × 1 + 5 × \(\frac{1}{10}\) + 2 × \(\frac{1}{100}\) = 3.52
• 4 × 10 + 6 × 1 + 32 × \(\frac{1}{100}\) = 46.32

How Big?
We already know that a hundred 10s make 1000, and a hundred 100s make 10000.

Notation, Writing, and Reading of Numbers
We have been writing numbers in a particular way, say 456, instead of writing them as 4 × 100 (4 hundreds) + 5 × 10 (5 tens) + 6 × 1 (6 ones). Similarly, can we skip writing tenths and hundredths?
Can the quantity 4\(\frac{2}{10}\) be written as 42 (skipping the \(\frac{1}{10}\) in 2 × \(\frac{1}{10}\))?
If yes, how would we know if 42 means 4 tens and 2 units or it means 4 units and 2 tenths?
Similarly, 705 could mean:

• 7 hundreds, 0 tens, and 5 ones (700 + 0 + 5)
• 7 tens and 0 units and 5 tenths (70 + 0 + \(\frac{5}{10}\))
• 7 units and 0 tenths and 5 hundredths (7 + \(\frac{0}{10}\) + \(\frac{5}{100}\))

Since these are different quantities, we need to have distinct ways of writing them.

To identify the place value where integers end and the fractional parts start, we use a point or period (‘.’) as a separator, called a decimal point. The above quantities in decimal notation are then:

These numbers, when shown through place value, are as follows:

Thus, decimal notation is a natural extension of the Indian place value system to numbers also having fractional parts. Just as 705 means 7 × 100 + 5 × 1, the number 70.5 means 7 × 10 + 5 × \(\frac{1}{10}\), and 7.05 means 7 × 1 + 5 × \(\frac{1}{100}\).

We have seen how to write numbers using the decimal point (‘.’). But how do we read/say these numbers?
We know that 705 is read as seven hundred and five.
70.5 is read as seventy point five, short for seventy and five-tenths.
7.05 is read as seven point zero five, short for seven and five hundredths.
0.274 is read as zero point two seven four. We don’t read it as zero point two hundred and seventy four, as 0.274 means 2 one-tenths and 7 one-hundredths and 4 one-thousandths.

In the chapter on large numbers, we learned how to write 23 hundred.
23 hundreds = 23 × 100 = 2000 + 300 = 2300.

Similarly, 23 tens would be:
23 tens = 23 × 10 = 200 + 30 = 230.

How can we write 234 tenths in decimal form?
234 tenths = \(\frac{234}{10}\)
= \(\frac{200}{10}+\frac{30}{10}+\frac{4}{10}\)
= 20 + 3 + \(\frac{4}{10}\)
= 23.4

Units of Measurement Class 7 Notes

Length Conversion
We have been using a scale to measure length for a few years. We already know that 1 cm = 10 mm (millimeters).

How many cm is 1 mm?
1 mm = \(\frac{1}{10}\) cm = 0.1 cm (i.e., one-tenth of a cm).

How many cm is (a) 5 mm? (b) 12 mm?
(a) 5 mm = \(\frac{5}{10}\) cm = 0.5 cm
(b) 12 mm = 10 mm + 2 mm
= 1 cm + \(\frac{2}{10}\) cm
= 1.2 cm

How many mm is 5.6 cm?
Since each cm has 10 mm, 5.6 cm (5 cm + 0.6 cm) is 56 mm.

The illustration below shows how small some things are! Try taking an approximate measurement of each.

→ The three blue stripes represent the typical relative sizes of pen strokes: fine stroke, medium stroke, and bold stroke.

→ A human hair is about 0.1 mm in thickness.

→ The thickness of a newspaper can range from 0.05 to 0.08 mm.

→ Mustard seeds have a thickness of 1 – 2 mm.

→ The smallest ant species discovered so far, Carabera Bruni, has a total length of 0.8 – 1 mm. They are found in Sri Lanka and China.

→ The smallest land snail species discovered so far, Acmella Nana, has a shell diameter of 0.7 mm. They are found in Malaysia.

We also know that 1 m = 100 cm. Based on this, we can say that
1 cm = \(\frac{1}{100}\) m = 0.01 m.

How many m is (a) 10 cm? (b) 15 cm?
(a) 10 cm = \(\frac{1}{10}\) m = 0.1 m
Since each cm is one-hundredth of a meter, 15 cm can be written as
(b) 15 cm = \(\frac{15}{100}\) m
= \(\frac{10}{100}\) m + \(\frac{5}{100}\) m
= \(\frac{1}{10}\) m + \(\frac{5}{100}\) m
= 0.15 m

Here, we have some more interesting facts about small things in nature!

→ The egg of a hummingbird typically is 1.3 cm long and 0.9 cm wide.

→ The Philippine Goby is about 0.9 cm long. It can be found in the Philippines and other Southeast Asian countries.

→ The smallest known jellyfish, Irukandji, has a bell size of 0.5 – 2.5 cm. Its tentacles can be as long as 1 m. They are found in Australia. Its venom can be fatal to humans.

→ The Wolf octopus, also known as the Star-sucker Pygmy Octopus, is the smallest known octopus in the world. Their typical size is around 1 – 2.5 cm, and they weigh less than 1 g. They are found in the Pacific Ocean.

• 1 mm = \(\frac{1}{10}\) cm = 0.1 cm; 1 cm = \(\frac{1}{100}\) m = 0.01 m
• 8 mm = 0.8 cm; 12 mm = 1.2 cm
• 4 cm = 0.04 m, 12 cm = 0.12 cm, 237 cm = 2.37 m

Weight Conversion
Let us look at kilograms (kg). We know that 1 kg = 1000 gram (g). We can say that
1 g = \(\frac{1}{1000}\) kg = 0.001 kg.

How many kilograms is 5 g?
5 g = \(\frac{5}{1000}\) kg = 0.005 kg.

How many kilograms is 10 g?
10 g = \(\frac{10}{1000}\) = \(\frac{1}{100}\) kg = 0.010 kg.
As each gram is one-thousandth of a kg, 254 g can be written as
254 g = \(\frac{254}{1000}\) kg
= (\(\frac{200}{1000}+\frac{50}{1000}+\frac{4}{1000}\)) kg
= (\(\frac{2}{10}+\frac{5}{100}+\frac{4}{1000}\)) kg
= 0.254 kg.

Look at the picture below showing different quantities of rice. Starting from the 1 g heap, subsequent heaps can be found that are 10 times heavier than the previous heap/packets. The combined weight of rice in this picture is 11.111 kg.

Also, 1 gram = 1000 milligrams (mg).
So, 1 mg = \(\frac{1}{1000}\) g = 0.001 g.

• 1 gram = \(\frac{1}{1000}\) kg = 0.001 kg
• 9 g = 0.009 kg, 42 g = 0.042 kg, 723 g = 0.723 kg, 4217 g = 4.217 kg

Rupee ─ Paise Conversion
You may have heard of ‘paisa’. 100 paise is equal to 1 rupee. As we have coins and notes for rupees, coins for paise were also used commonly until recently. There were coins for 1 paisa, 2 paise, 3 paise, 5 paise, 10 paise, 20 paise, 25 paise, and 50 paise. All denominations of 25 paise and less were removed from use in the year 2011. But we still see paisa in bills, account statements, etc.

1 rupee = 100 paise
1 paisa = \(\frac{1}{100}\) rupee = 0.01 rupee
As each paisa is one-hundredth of a rupee,
75 paise = \(\frac{75}{100}\) rupee
= (\(\frac{70}{100}+\frac{5}{100}\)) rupee
= (\(\frac{7}{10}+\frac{5}{100}\)) rupee
= 0.75 rupee.

During the 1970s, a masala dosa cost just 50 paise, one could buy a banana for 20-25 paise, a handful of peppermints were available for 2 paise or 3 paise, and a kg of rice cost ₹ 2.45.3

1. 1 paisa = \(\frac{1}{100}\) rupee = ₹ 0.01
2. 5 paise = ₹ 0.05, 12 paise = ₹ 0.12, 564 paise = ₹ 5.64

Locating and Comparing Decimals

Let us consider the decimal number 1.4. It is equal to 1 unit and 4 tenths. This means that the unit between 1 and 2 is divided into 10 equal parts, and 4 such parts are taken. Hence, 1.4 lies between 1 and 2. Draw the number line and divide the unit between 1 and 2 into 10 equal parts. Take the fourth part, and we have 1.4 on the number line.

Let us consider the decimal number 1.4. It is equal to 1 unit and 4 tenths.
This means that the unit between 1 and 2 is divided into 10 equal parts, and 4 such parts are taken. Hence, 1.4 lies between 1 and 2.
We draw the number line and divide the unit between 1 and 2 into 10 equal parts.

Comparing

Let us compare 4.62 and 5.71. 4.62 has 4 ones. 5.71 has 5 ones. 4 <5. Hence 4.62 < 5.71.

Let us compare 7.89 and 7.671. Both have the same number of ones. So we compare the number of tenths. 7.89 has 8-tenths. 7.671 has 6-tenths. 8 > 6. Hence, 7.89 > 7.671.

There is Zero Dilemma!
Sonu says that 0.2 can also be written as 0.20, 0.200; Zara thinks that putting zeros on the right side may alter the value of the decimal number. What do you think?
We can figure this out by looking at the quantities these numbers represent using place value.

We can see that 0.2, 0.20, and 0.200 are all equal as they represent the same quantity, i.e., 2 tenths. But 0.2, 0.02, and 0.002 are different.

In the number line shown below, what decimal numbers do the boxes labelled ‘a’, ‘b’, and ‘c’ denote?

The box with ‘b’ corresponds to the decimal number 7.5; are you able to see how? There are 5 units between 5 and 10, divided into 10 equal parts. Hence, every 2 divisions make a unit, and so every division is \(\frac{1}{2}\) unit. What numbers do ‘a’ ‘c’ denote?

Which is larger: 6.456 or 6.465?
To answer this, we can use the number line to locate both decimal numbers and show which is larger. This can also be done by comparing the corresponding digits at each place value, as we do with whole numbers. This comparison is visualised step by step below. Note that the visualisation below is not to scale.

We start by comparing the most significant digits (digits with the highest place value) of the two numbers. If the digits are the same, we compare the next smaller place value. We keep going till we find a position where the digits are not equal. The number with the larger digit at this position is the greater of the two.

0.3 = 0.30 – 0.300; 0.03 = 0.030
Putting zeroes on the right side does not alter the value of the decimal.

Closest Decimals
Consider the decimal numbers 0.9, 1.1, 1.01, and 1.11. Identify the decimal number that is closest to 1.
Let us compare the decimal numbers. Arranging these in ascending order, we get 0.9 < 1 < 1.01 < 1.1 < 1.11.
Among the neighbours of 1, 1.01 is \(\frac{1}{100}\) away from 1, whereas 0.9 is \(\frac{10}{100}\) away from 1.
Therefore, 1.01 is closest to 1.

Consider the decimals: 2.7, 2.6, 2.2, 2.01
The closest to 2.5 is 2.6.
The closest to 2.1 is 2.01 etc.

Addition and Subtraction of Decimals Class 7 Notes

Priya requires 2.7 m of cloth for her skirt, and Shylaja requires 3.5m for her kurti. What is the total quantity of cloth needed?
We have to find the sum of 2.7 m + 3.5 m.
Earlier, we saw how to add 2\(\frac{7}{10}\) + 3\(\frac{5}{10}\) (also shown below). Can you carry out the same addition using decimal notation? It is shown below. Share your observations.
The total quantity of cloth needed is 6.2 m.

How much longer is Shylaja’s cloth compared to Priya’s?
We have to find the difference between 3.5 m – 2.7 m.
Again, observe how the diffrences 3\(\frac{5}{10}\) – 2\(\frac{7}{10}\) and 3.5 m – 2.7 m are computed.

As you can see, the standard procedure for adding and subtracting whole numbers can be used to add and subtract decimals.

A detailed view of the underlying place value calculation is shown below for the sum 75.345 + 86.691. Its compact form is shown next to it.

Any two or more decimals can be added.
Also, a decimal number can be subtracted from a larger decimal number.
In vertical addition and subtraction, decimal must be placed under the decimal.

Decimal Sequences
Observe this sequence of decimal numbers and identify the change after each term.
4.4, 4.8. 5.2, 5.6, 6.0, ……
We can see that 0.4 is being added to a term to get the next term.

4.2, 4.5, 4.8, 5.1, ………..
Note that each term exceeds the preceding term by 0.3.
Such sequences are called decimal sequences.

Estimating Sums and Differences
Sonu has observed sums and differences of decimal numbers and says, “If we add two decimal numbers, then the sum will always be greater than the sum of their whole number parts. Also, the sum will always be less than 2 more than the sum of their whole number parts.”

Let us use an example to understand what his claim means: If the two numbers to be added are 25.936 and 8.202, the claim is that their sum will be greater than 25 + 8 (whole number parts) and will be less than 25 + 1 + 8 + 1.

1. The sum of two decimal numbers will be more than the sum of their whole number parts. Also, it will be less than 2 more than the sum of their whole number parts.
12.36 + 14.98 will be more than 12 + 14 or 28.
Also, it will be less than 12 + 14 + 2 or 28.

2. The difference of two decimal numbers will be more than the difference of their whole number parts decreased by 1.
Also, it will be less than the difference of their whole number parts increased by 1.
14.68 – 11.94 will be more than 14 – 11 – 1 or 2.
Also, it will be less than 14 – 11 + 1 or 4.

More on the Decimal System Class 7 Notes

Decimal and Measurement Disasters
Decimal point and unit conversion mistakes may seem minor sometimes, but they can lead to serious problems. Here are some actual incidents in which such errors caused major issues.

→ In 2013, the finance office of the Amsterdam City Council (Netherlands) mistakenly sent out €188 million in housing benefits instead of the intended €1.8 million due to a programming error that processed payments in euro cents instead of euros. (1 euro-cent = \(\frac{1}{100}\) euro).

→ In 1983, a decimal error nearly caused a disaster for an Air Canada Boeing 767. The ground staff miscalculated the fuel, loading 22,300 pounds instead of kilograms—about half of what was needed (1 pound ~ 0.453 kg). The plane ran out of fuel mid-air, forcing the pilots to make an emergency landing at an abandoned airfield. Fortunately, everyone survived.

Several incidents have occurred due to the incorrect reading of decimal numbers while giving medication. For example, reading 0.05 mg as 0.5 mg can lead to using a medicine 10 times more than the prescribed quantity. It is therefore important to pay attention to units and the location of the decimal point.

Decimal and measurement disasters likely refer to common mistakes and misunderstandings when working with decimals and measurements. These could include misinterpreting decimal place values, incorrectly converting between units of measurement (like centimeters and millimeters), or struggling with operations like addition and subtraction involving decimals and mixed units.

Deceptive Decimal Notation
Sarayu gets a message: “The bus will reach the station 4.5 hours post noon.” When will the bus reach the station: 4:05 p.m., 4:50 p.m., 4:25 p.m.? None of these! Here, 0.5 hours means splitting an hour into 10 equal parts and taking 5 parts out of it. Each part will be 6 minutes (60 minutes/10) long. 5 such parts make 30 minutes. So, the bus will reach the station at 4:30.

Here is a short story of a decimal mishap: A girl measures the width of an opening as 2 ft 5 inches but conveys to the carpenter to make a door 2.5 ft wide. The carpenter makes a door of width 2 ft 6 inches (since 1 ft = 12 inches, 0.5 ft = 6 inches), and it wouldn’t close fully

If you watch cricket, you might have noticed decimal-looking numbers like ‘Overs left: 5.5’. Does this mean 5 overs and 5 balls or 5 overs and 3 balls? Here, 5.5 overs means 5\(\frac{5}{6}\) overs (as 1 over = 6 balls), i.e., 5 overs and 5 balls.

A Pinch of History – Decimal Notation Over Time
Decimal fractions (i.e., fractions with denominators like \(\frac{1}{10}\), \(\frac{1}{100}\), \(\frac{1}{1000}\), and so on) are used in the works of several ancient Indian astronomers and mathematicians, including in the important 8th century works of Shridharacharya on arithmetic and algebra. Decimal notation, in essentially its modern form, was described in detail in Kitab al-Fusul fi al-Hisab al Hindi (The Book of Chapters on Indian Arithmetic) by Abul Hassan al-Uqlidisi, an Arab mathematician, in around 950 CE. He represented the number 0.059375 as 0.059375.

In the 15th century, to separate whole numbers from fractional parts, several different notations were used:

• a vertical mark on the last digit of the whole number part (as shown above),
• Use of different colours and
• numerical superscript giving the number of fractional decimal places (0.36 would be written as 362 ).

In the 16th century, John Napier, a Scottish mathematician, and Christopher Clavius, a German mathematician, used the point/period (‘.’) to separate the whole number and the fractional parts, while François Viète, a French mathematician, used the comma (‘,’) instead.

Currently, several countries use the comma to separate the integer part and the fractional part. In these countries, the number 1,000.5 is written as 1000,5 (space as a thousand separator). But the decimal point has endured as the most popular notation for writing numbers having fractional parts in the Indian place value system.

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