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

©All rights reserved. onlinemath4all.com