Singapore Math Level 3A & 3B – Unit 9: Mental Calculations - 40 Questions
Unit 9: MENTAL CALCULATIONS
Friends, in the Mental Calculations method, we have provided solutions to all the addition, subtraction, multiplication, and division problems in Unit 9 of Singapore Math Level 3A. Let’s try to understand the explanation with an example. Suppose we have to add 13 and 25, i.e., 25 + 13 = ? Let’s see how to solve it using the Mental Calculations method.
25 + 13
[Divide one of the two numbers into two convenient parts in such a way that it is easy to add one of the divided parts to the other number. For example, divide 13 into 10 and 3 and write it as follows:]
= 25 + 10 + 3
= 35 + 3 [Add 10 and 25.]
= 38 [Add the remaining part to the sum to get the final result.]
In this way, we follow some simple rules for addition, subtraction, multiplication, and division using Mental Calculations method.
Q(1-40): Solve the following problems mentally.
1. 65 + 24
= 65 + 20 + 4
= 85 + 4
= 89
2. 39 + 13
= 39 + 10 + 3
= 49 + 3
= 49 + 1 + 2
= 50 + 2
= 52
Or,
39 + 13
= 40 + 13 – 1
= 53 – 1
= 52
3. 56 + 78
= 56 + 80 – 2
= 136 – 2
= 134
4. 84 + 44
= 84 + 40 + 4
= 124 + 4
= 128
5. 74 + 25
= 74 + 20 + 5
= 94 + 5
= 99
6. 31 + 87
= 31 + 80 + 7
= 111 + 7
= 118
7. 42 + 48
= 42 + 50 – 2
= 92 – 2
= 90
8. 61 + 97
= 61 + 100 – 3
= 161 – 3
= 158
9. 57 + 29
= 57 + 30 – 1
= 87 – 1
= 86
10. 19 + 58
= 20 + 58 – 1
= 78 – 1
= 77
11. 49 – 18
= 49 – 10 – 8
= 39 – 8
= 31
or,
49 – 18
= 49 – 20 + 2
= 29 + 2
= 31
12. 74 – 53
= 74 – 50 – 3
= 24 – 3
= 21
13. 64 – 19
= 64 – 20 + 1
= 44 + 1
= 45
14. 83 – 28
= 83 – 30 + 2
= 53 + 2
= 55
15. 37 – 9
= 37 – 10 + 1
= 27 + 1
= 28
16. 86 – 47
= 86 – 50 + 3
= 36 + 3
= 39
17. 62 – 35
= 62 – 40 + 5
= 22 + 5
= 27
18. 77 – 46
= 77 – 50 + 4
= 27 + 4
= 31
19. 96 – 47
= 96 – 50 + 3
= 46 + 3
= 49
20. 55 – 27
= 55 – 30 + 3
= 25 + 3
= 28
21. 5 × 8
= 40
22. 9 × 9
= 81
23. 7 × 3
= 21
24. 3 × 6
= 18
25. 4 × 7
= 28
26. 2 × 9
= 18
27. 8 × 60
= 8 × 6 tens
= 48 tens
= 480
28. 6 × 70
= 6 × 7 tens
= 42 tens
= 420
29. 3 × 400
= 3 × 4 hundreds
= 12 hundreds
= 1200
30. 5 × 500
= 5 × 5 hundreds
= 25 hundreds
= 2500
31. 24 ÷ 3
= 8
32. 54 ÷ 6
= 9
33. 50 ÷ 5
= 10
34. 72 ÷ 8
= 9
35. 90 ÷ 9
= 10
36. 80 ÷ 4
= 8 tens ÷ 4
= 2 tens
= 20
37. 350 ÷ 7
= 35 tens ÷ 7
= 5 tens
= 50
38. 210 ÷ 3
= 21 tens ÷ 3
= 7 tens
= 70
39. 360 ÷ 6
= 36 tens ÷ 6
= 6 tens
= 60
40. 160 ÷ 4
= 16 tens ÷ 4
= 4 tens
= 4 tens
= 40
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