Singapore Math Level 2A & 2B – Unit 10: Mental Calculations
Addition and subtraction can be done by two simple mental calculations. They are (a) rounding numbers and (b) breaking up numbers.
Mental addition calculation by rounding numbers:
1. Round one of the addends, A, to the nearest ten.
2. Mentally add the rounded number to the other addend, B.
3. Subtract the difference between the rounded number and addend A from the sum.
Mental addition calculation by breaking up numbers:
When one of the addends is less than 10,
1. break up the other addend into ones and tens/hundreds. Example: Break up 476 into 6 and 470.
2. Add the ones to get a sum.
3. Add the sum to the remaining tens/hundreds to get the final answer.
Go ahead with the same process when an addend is less than 100 or 1000.
Mental subtraction calculation by rounding numbers:
1. Round one of the subtrahends to the nearest ten.
2. Mentally subtract the rounded number from the other subtrahend.
3. Add the difference between the rounded number and the first subtrahend to the result in step 2.
Mental subtraction calculation by breaking up numbers:
When one of the subtrahends is less than 10 then,
1. break up the other subtrahend into ones and tens or hundreds. Example: Break up 738 into 8 and 730.
2. Subtract the ones to get a result.
3. Add the result in step 2 to the remaining tens or hundreds.
Go ahead with the same process when a subtrahend is less than 100 or 1000.
1. What is 69 + 7?
69 + 10 = 79
79 – 3 = 76
69 + 7 = 76
2. What is 364 + 5?
364 = 360 + 4
4 + 5 = 9
360 + 9 = 369
364 + 5 = 369
3. What is 158 + 30?
158 = 108 + 50
50 + 30 = 80
108 + 80 = 188
158 + 30 = 188
4. What is 592 + 400?
592 = 500 + 92
500 + 400 = 900
900 + 92 = 992
592 + 400 = 992
5. What is 95 – 9?
95 – 10 = 85
85 + 1 = 86
95 – 9 = 86
6. What is 438 – 60?
438 – 100 = 338
338 + 40 = 378
438 – 60 = 378
7. What is 247 – 40?
247 = 207 + 40
40 – 40 = 0
207 + 0 = 207
247 – 40 = 207
8. What is 689 – 300?
689 = 600 + 89
600 – 300 = 300
300 + 89 = 389
689 – 300 = 389
Solve the following addition problems mentally.
1. 64+8= ————————–
Solution:
64+10=74
74-2=72
2. 89+7= ————————–
Solution:
89+10=99
99-3=96
3. 26+5= ————————–
Solution:
26+10=36
36-5=31
4. 18+9= ————————–
Solution:
18+10=28
28-1=27
5. 57+6= ————————–
Solution:
57+10=67
67-4=63
6. 45+8=————————–
Solution:
45+10=55
55-2=53
7. 37+5=————————–
Solution:
37+10=47
47-5=42
8. 78+8=————————–
Solution:
78+10=88
88-2=86
9. 94+9=————————–
Solution:
94+10=104
104-1=103
10. 56+7=————————–
Solution:
56+10=66
66-3=63
11. 127+5=————————–
Solution:
127+10=137
137-5=132
12. 764+9=————————–
Solution:
764+10=774
774-1=773
13. 262+6=————————–
Solution:
2+6=8
260+8=268
14. 948+8=————————–
Solution:
948+10=958
958-2=956
15. 435+7=————————–
Solution:
435+10=445
445-3=442
16. 584+6=————————–
Solution:
4+6=10
580+10=590
17. 623+9=————————–
Solution:
623+10=633
633-1=632
18. 806+9=————————–
Solution:
806+10=816
816-1=815
19. 366+5=————————–
Solution:
366+10=376
376-5=371
20. 119+6=————————–
Solution:
119+10=129
129-4=125
21. 513+6=————————–
Solution:
3+6=9
510+9=519
22. 836+20=————————–
Solution:
30+20=50
806+50=856
23. 723+80=————————–
Solution:
20+80=100
703+100=803
24. 190+70=————————–
Solution:
90+70=160
100+160=260
25. 428+40=————————–
Solution:
20+40=60
408+60=468
26. 762+70=————————–
Solution:
762+100=862
862-30=832
27. 503+90=————————–
Solution:
503+100=603
603-10=593
28. 869+80=————————–
Solution:
869+100=969
969-20=949
29. 623+60=————————–
Solution:
20+60=80
603+80=683
30. 770+200=————————–
Solution:
700+200=900
900+70=970
31. 323+600=————————–
Solution:
300+600=900
900+23=923
32. 165+800=————————–
Solution:
100+800=900
900+65=965
33. 248+500=————————–
Solution:
200+500=700
700+48=748
34. 657+300=————————–
Solution:
600+300=900
900+57=957
35. 195+700=————————–
Solution:
100+700=800
800+95=895
36. 108+200=————————–
Solution:
100+200=300
300+8=308
37. 588+400=————————–
Solution:
500+400=900
900+88=988
38. 645+100=————————–
Solution:
600+100=700
700+45=745
39. 199+600=————————–
Solution:
100+600=700
700+99=799
40. 756+200=————————–
Solution:
700+200=900
900+56=956
Solve the following subtraction problems mentally
41. 52–5=————————–
Solution:
52-10=42
42+5=47
42. 46–9=————————–
Solution:
46-10=36
36+1=37
43. 81–8=————————–
Solution:
81-10=71
71+2=73
44. 30–7=————————–
Solution:
30-10=20
20+3=23
45. 88–3=————————–
Solution:
8-3=5
80+5=85
46. 79–5=————————–
Solution:
9-5=4
70+4=74
47. 64–4=————————–
Solution:
4-4=0
60+0=60
48. 28–9=————————–
Solution:
28-10=18
18+1=19
49. 93–1=————————–
Solution:
3-1=2
90+2=92
50. 59–7=————————–
Solution:
9-7=2
50+2=52
51. 620–5=————————–
Solution:
620-10=610
610+5=615
52. 404–6=————————–
Solution:
404-10=394
394+4=398
53. 875–4 =————————–
Solution:
5-4=1
870+1=871
54. 740–2=————————–
Solution:
10-2=8
730+8=738
55. 519–9=————————–
Solution:
9-9=0
510+0=510
56. 264–7=————————–
Solution:
264-10=254
254+3=257
57. 329–6=————————–
Solution:
9-6=3
320+3=323
58. 183–5=————————–
Solution:
183-10=173
173+5=178
59. 916–3=————————–
Solution:
6-3=3
910+3=913
60. 534–8=————————–
Solution:
534-10=524
524+2=526
61. 415–30=————————–
Solution:
415-100=315
315+70=385
62. 338–90=————————–
Solution:
338-100=238
238+10=248
63. 587–60=————————–
Solution:
80-60=20
507+20=527
64. 860–50=————————–
Solution:
60-50=10
800+10=810
65. 609–10=————————–
Solution:
100-10=90
509+90=599
66. 281–20=————————–
Solution:
80-20=60
201+60=261
67. 758–40=————————–
Solution:
50-40=10
708+10=718
68. 495–70=————————–
Solution:
90-70=20
405+20=425
69. 164–80=————————–
Solution:
164-100=64
64+20=84
or,
100-80=20
64+20=84
70. 626–60=————————–
Solution:
100-60=40
526+40=566
or,
626-100=526
526+40=566
71. 758–300=————————–
Solution:
700-300=400
400+58=458
72. 834–600=————————–
Solution:
800-600=200
200+34=234
73. 905–800=————————–
Solution:
900-800=100
100+5=105
74. 631–500=————————–
Solution:
600-500=100
100+31=131
75. 978–900=————————–
Solution:
900-900=0
78+0=78
76. 505–100=————————–
Solution:
500-100=400
400+5=405
77. 784–400=————————–
Solution:
700-400=300
300+84=384
78. 435–200=————————–
Solution:
400-200=200
200+35=235
79. 876–700=————————–
Solution:
800-700=100
100+76=176
80. 980–800=————————–
Solution:
900-800=100
100+80=180
Singapore Math Level 2A & 2B – Unit 10 – Mental Calculations
