We can add two numbers mentally by changing one of the numbers to a ten and then adjusting the same from the other number. 

This method is known as Compensation Strategy for Mental Addition.


Example:  Add 38 and 24 mentally.

CASE-1



CASE-2



CASE-3



CASE-4



We can see that, in all the four cases above, the sum is 62.

In CASE-1, the number 38 is changed into a ten by addition.

In CASE-2, the number 24 is changed into a ten by addition.

In CASE-3, the number 38 is changed into a ten by subtraction.

In CASE-4, the number 24 is changed into a ten by subtraction.

We can add two numbers mentally in any of these four ways.

QUESTIONS FOR PRACTICE
Question 1.
Change the first number into a ten by addition and then add:
(a) 46 and 27               (b) 34 and 49                   (c) 76 and 15

Question 2.
Change the second number into a ten by addition and then add:
(a) 46 and 27               (b) 34 and 49                   (c) 76 and 15

Question 3.
Change the first number into a ten by subtraction and then add:
(a) 46 and 27               (b) 34 and 49                   (c) 76 and 15

Question 4.
Change the second number into a ten by subtraction and then add:
(a) 46 and 27               (b) 34 and 49                   (c) 76 and 15