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.

Video Lesson

Solved Problems

In each case, evaluate the expression.

Problem 1 :

43 - 4

Solution :

= 43 - 4

= 40 + 3 - 3 - 1

= 40 - 1

= 39

Problem 2 :

52 - 6

Solution :

= 52 - 6

= 52 - 2 - 4

= 50 - 4

= 46

Problem 3 :

47 - 9

Solution :

= 47 - 9

= 47 - 10 + 1

= 37 + 1

= 38

Problem 4 :

64 - 8

Solution :

= 64 - 8

= 64 - 10 + 2

= 54 + 2

= 56

Problem 5 :

38 - 23

Solution :

= 38 - 23

= 38 - 20 + 3

= 18 + 3

= 21

Problem 6 :

47 - 19

Solution :

= 47 - 19

= 47 - 20 + 1

= 27 + 1

= 28

Problem 7 :

54 - 29

Solution :

= 54 - 29

= 54 - 30 + 1

= 24 + 1

= 25

Problem 8 :

358 - 14

Solution :

= 358 - 14

= 358 - 10 - 4

= 348 - 4

= 344

Problem 9 :

786 - 15

Solution :

= 786 - 15

= 786 - 10 - 5

= 776 - 5

= 771

Problem 10 :

276 - 39

Solution :

= 276 - 39

= 276 - 40 + 1

= 236 + 1

= 237

Problem 11 :

346 - 145

Solution :

= 346 - 145

= (300 - 100) + (46 - 45)

= 200 + 1

= 201

Problem 12 :

456 - 272

Solution :

= 456 - 272

= (400 - 200) + (56 - 72)

= 200 - 16

= 184

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