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Answers for Mental subtraction worksheet

It is important to note that these suggested answers are just one of the many possible strategies that children may use.

1.

300 - 10 = 290
Possible strategy: renaming
Rename 300 to 30 tens and 10 to 1 ten. 1 ten less than 30 tens is 29 tens. 29 tens is equal to 290.

2.

90 - 18 = 72
Possible strategy: subtraction in stages
90 - 10 = 80
80 - 8 = 72

3.

35 - 6 = 29
Possible strategy: subtraction in stages
35 - 5 = 30
30 - 1 = 29

4

92 - 58 = 34
Possible strategy: equal addition to change 58 to 60
92 - 58
= (92 + 2) - (58 + 2)
= 94 - 60
= 34

5.

100 - 42 = 58
Possible strategy: complementary addition
Firstly, 8 is added to 42 to give 50. Then another 50 is added to give 100. This is a total of 52 added to 42 to give 100.

6.

200 - 101 = 99
Possible strategy: subtraction in stages
200 - 100 = 100
100 - 1 = 99

For the following questions 7, 8 and 9, discuss the questions together and note the differences between each.

7.

444 - 102 = 342
Possible strategy: subtraction in stages
444 - 100 = 344
344 - 2 = 342

8.

555 - 99 = 456
Possible strategy: rounding
555 - 100 + 1
= 455 + 1
= 456

9.

666 - 97 = 569
Possible strategy: rounding
666 - 100 + 3
= 566 + 3
= 569

10.

3681 - 600 = 3081
Possible strategy: renaming
Rename 3681 to 36 hundreds and 81 ones. 36 hundreds subtract 6 hundreds gives 30 hundreds. Therefore the answer is 30 hundreds and 81 ones or 3081.

11.

33 - 18 = 15
Possible strategy: complementary addition
Firstly, add 2 to 18 to give 20. Then add 10 to 20 to give 30. Finally add 3 to 30 to give 33. 2 + 10 + 3 has been added to 18 to give 33 therefore, the difference between the two amounts is $15.

12.

82 - 16 = 66
Possible strategy: subtraction in stages
82 - 10 = 72
72 - 6 = 66

13.

67 - 19 = 48
Possible strategy: complementary addition
Firstly, add 1 to 19 to give 20. Then add 40 to 20 to get to 60 and finally add a finally 7 to give 67. Altogether 48 has been added.

14.

100 - 38 = 62
Possible strategy: rounding
100 - 40 + 2
= 60 + 2
= 62

15.

7.45 - 1.95 = 5.50
Possible strategy: equal addition
 ($7.45 + 5c) - ($1.95 + 5c)
= $7.50 - $2
= $5.50
It is often the case that students will find mental computation easier when there is something concrete (such as money) to visualize. Note that most students will do this question as a whole numbers question rather than as a decimal question.

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