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
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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