Mental Arithmetic: add and subtract multiples of 10 and 100 mentally

Mental Arithmetic

Able to add and subtract multiples of 10 and 100 mentally.

Multiples of 10 and 100 are useful in counting large number of items and objects (including money).

Multiples of 10

10, 20, 30, 40, 50 . . ..

Adding with multiples of 10

Here, one’s place didn’t change, when they are added to zero and hundreds Place also didn’t change, only the tens place got changed.

i) 2 + 1 = 3

ii) 4 + 1 = 5

Now we add the following

EXAMPLE 1

Add 374 + 10

4 + 0 = 4

7 + 1 = 8

Therefore 374 + 10 = 384

EXAMPLE 2

Add 286 + 30

8 + 3 = 11

Here, 11 has two digits, in this keep 1 in the tens place, take the other 1 and add with the hundreds place digit.

Therefore 286 + 30 = 316

Adding with multiples of 100

Here, one's place and ten's place didn't change, when they are added to zero but in the hundreds place it is changed.

EXAMPLE 3

Add 682 + 100

2 + 0 = 2

8 + 0 = 8

6 + 1 = 7

∴ 682 + 100 = 782

EXAMPLE 4

Add 835 with 100

8 + 1 = 9

∴ 835 + 100 = 935

Subtracting by multiples of 10

Follow the same procedure of addition for subtraction also, instead of adding the numbers, subtract the numbers in the circle.

EXAMPLE 5

Subtract 10 from 625

5 – 0 = 5

2 – 1 = 1

∴ 625 – 10 = 615

EXAMPLE 6

Subtract 50 from 981

8 – 5 = 3

∴ 981 – 50 = 931

Subtracting by multiples of 100

EXAMPLE 7

Subtract 400 from 546

5 – 4 = 1

∴ 546 – 400 = 146