ADDING TWO DIGIT NUMBERS MENTALLY

Round Figure of a Two-Digit Number :

A two-digit number will be considered as a round figure, if it has zero at ones place.

Examples :

10, 20, 50, 90

When two digit numbers are round figures, we can add them easily. 

30 + 50 = 80

To add 30 and 80, first we have to add 3 and 5, which is 8. Then, take zero along with 8, then it is 80.

The basic idea of adding two-digits numbers mentally is to break the two-digit numbers into tens and ones.

For example, 63 can be written as (60 + 3).

Let us see, how to add 56 and 22.

= 56 + 22

Break each two-digit number into tens and ones.

= 50 + 6 + 20 + 2

Group the round figures and singe-digit numbers.

= (50 + 20) + (6 + 2)

= 70 + 8

= 78

When you get a two-digit number like 19 or 28, we can write them as

19 = 20 - 1

28 = 30 + 2

Since 19 is very close to the round figure 20, instead of writing 19 as (10 + 9), we can write it as (20 - 1), it will make the calculation process easier.

Let us see, how to add 74 and 19.

= 74 + 19

= 70 + 4 + 20 - 1

= (70 + 20) + (4 - 1)

= 90 + 3

= 93

Video Lesson

Solve Problems

In each case, evaluate the expression.

Problem 1 :

23 + 45

Solution :

= 23 + 45

= 20 + 3 + 40 + 5

= (20 + 40) + (3 + 5)

= 60 + 8

= 68

Problem 2 :

54 + 73

Solution :

= 54 + 73

= 50 + 4 + 70 + 3

= (50 + 70) + (4 + 3)

= 120 + 7

= 127

Problem 3 :

27 + 41

Solution :

= 27 + 41

= 20 + 7 + 40 + 1

= (20 + 40) + (7 + 1)

= 60 + 8

= 68

Problem 4 :

32 + 79

Solution :

= 32 + 79

= 30 + 2 + 80 - 1

= (30 + 80) + (2 - 1)

= 110 + 1

= 111

Problem 5 :

40 + 85

Solution :

= 40 + 85

= 40 + 80 + 5

= (40 + 80) + 5

= 120 + 5

= 125

Problem 6 :

49 + 98

Solution :

= 49 + 98

= 50 - 1 + 100 - 2

= (50 + 100) + (-1 - 2)

= 150 - 3

= 147

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