MENTAL MATH SUBTRACTION

Round Figure :

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

Examples :

10, 20, 50

A three-digit number will be considered as round figure, if it has zeros at tens place and ones place.

Examples :

100, 200, 600

Subtracting a One-Digit Number From a Two-Digit Number

Method 1 :

Consider the following subtraction :

13 - 7

What number has to be subtracted from 13, so that, it will become a round figure?

If we subtract 3 from 13, it will become 10, which is a round figure.

Now, write 13 as (10 + 3) and -7 as (-3 - 4).

= 13 - 7

= 10 + 3 - 3 - 4

= 10 - 4

= 6

The strategy is to remove a value from the two-digit number such that it will become a round figure and do the rest as explained in the above example.

Method 2 :

Incase, the one-digit number is very close to a round figure 10, we can do the subtraction as follows.

Consider the following subtraction :

28 - 9

Here, 9 is very close to the round figure 10. So, we can write -9 as (-10 + 1).

= 28 - 9

= 28 - 10 + 1

= 18 + 1

= 19

Subtracting a Two-Digit Number From a Two-Digit Number

Method 1 :

Consider the following subtraction :

38 - 23

Break the second two-digit number 23 into tens and ones. That is, write -23 as (-20 - 3).

Because, subtracting round figure 20 from 38 is easy. Then, subtract 3 from the result.

= 38 - 23

= 38 - 20 - 3

= 18 - 3

= 15

The strategy is to write the second two-digit number in terms of a round figure and do the rest as explained in the above example.

Method 2 :

Incase, the second-digit number is very close to a round figure, we can do the subtraction as follows.

Consider the following subtraction :

43 - 18

Here, 18 is very close to the round figure 20. So, we can write -18 as (-20 + 2).

= 43 - 18

= 43 - 20 + 2

= 23 + 2

= 25

We can apply the same two methods to subtract a two-digit number from a three-digit number mentally.

©All rights reserved. onlinemath4all.com