Singapore Math Level 2A & 2B – Unit 10: Mental Calculations - 80 Questions [Exact]
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. 64+8=_____
Solution(1):
64+10=74
74-2=72
2. 89+7=_____
Solution(2):
89+10=99
99-3=96
3. 26+5=_____
Solution(3):
26+10=36
36-5=31
4. 18+9=_____
Solution(4):
18+10=28
28-1=27
5. 57+6=_____
Solution(5):
57+10=67
67-4=63
6. 45+8=_____
Solution(6):
45+10=55
55-2=53
7. 37+5=_____
Solution(7):
37+10=47
47-5=42
8. 78+8=_____
Solution(8):
78+10=88
88-2=86
9. 94+9=_____
Solution(9):
94+10=104
104-1=103
10. 56+7=_____
Solution(10):
56+10=66
66-3=63
11. 127+5=_____
Solution(11):
127+10=137
137-5=132
12. 764+9=_____
Solution(12):
764+10=774
774-1=773
13. 262+6=_____
Solution(13):
2+6=8
260+8=268
14. 948+8=_____
Solution(14):
948+10=958
958-2=956
15. 435+7=_____
Solution(15):
435+10=445
445-3=442
16. 584+6=_____
Solution(16):
4+6=10
580+10=590
17. 623+9=_____
Solution(17):
623+10=633
633-1=632
18. 806+9=_____
Solution(18):
806+10=816
816-1=815
19. 366+5=_____
Solution(19):
366+10=376
376-5=371
20. 119+6=_____
Solution(20):
119+10=129
129-4=125
21. 513+6=_____
Solution(21):
3+6=9
510+9=519
22. 836+20=_____
Solution(22):
30+20=50
806+50=856
23. 723+80=_____
Solution(23):
20+80=100
703+100=803
24. 190+70=_____
Solution(24):
90+70=160
100+160=260
25. 428+40=_____
Solution(25):
20+40=60
408+60=468
26. 762+70=_____
Solution(26):
762+100=862
862-30=832
27. 503+90=_____
Solution(27):
503+100=603
603-10=593
28. 869+80=_____
Solution(28):
869+100=969
969-20=949
29. 623+60=_____
Solution(29):
20+60=80
603+80=683
30. 770+200=_____
Solution(30):
700+200=900
900+70=970
31. 323+600=_____
Solution(31):
300+600=900
900+23=923
32. 165+800=_____
Solution(32):
100+800=900
900+65=965
33. 248+500=_____
Solution(33):
200+500=700
700+48=748
34. 657+300=_____
Solution(34):
600+300=900
900+57=957
35. 195+700=_____
Solution(35):
100+700=800
800+95=895
36. 108+200=_____
Solution(36):
100+200=300
300+8=308
37. 588+400=_____
Solution(37):
500+400=900
900+88=988
38. 645+100=_____
Solution(38):
600+100=700
700+45=745
39. 199+600=_____
Solution(39):
100+600=700
700+99=799
40. 756+200=_____
Solution(40):
700+200=900
900+56=956
Solve the following subtraction problems mentally
41. 52–5=_____
Solution(41):
52-10=42
42+5=47
42. 46–9=_____
Solution(42):
46-10=36
36+1=37
43. 81–8=_____
Solution(43):
81-10=71
71+2=73
44. 30–7=_____
Solution(44):
30-10=20
20+3=23
45. 88–3=_____
Solution(45):
8-3=5
80+5=85
46. 79–5=_____
Solution(46):
9-5=4
70+4=74
47. 64–4=_____
Solution(47):
4-4=0
60+0=60
48. 28–9=_____
Solution(48):
28-10=18
18+1=19
49. 93–1=_____
Solution(49):
3-1=2
90+2=92
50. 59–7=_____
Solution(50):
9-7=2
50+2=52
51. 620–5=_____
Solution(51):
620-10=610
610+5=615
52. 404–6=_____
Solution(52):
404-10=394
394+4=398
53. 875–4 =_____
Solution(53):
5-4=1
870+1=871
54. 740–2=_____
Solution(54):
10-2=8
730+8=738
55. 519–9=_____
Solution(55):
9-9=0
510+0=510
56. 264–7=_____
Solution(56):
264-10=254
254+3=257
57. 329–6=_____
Solution(57):
9-6=3
320+3=323
58. 183–5=_____
Solution(58):
183-10=173
173+5=178
59. 916–3=_____
Solution(59):
6-3=3
910+3=913
60. 534–8=_____
Solution(60):
534-10=524
524+2=526
61. 415–30=_____
Solution(61):
415-100=315
315+70=385
62. 338–90=_____
Solution(62):
338-100=238
238+10=248
63. 587–60=_____
Solution(63):
80-60=20
507+20=527
64. 860–50=_____
Solution(64):
60-50=10
800+10=810
65. 609–10=_____
Solution(65):
100-10=90
509+90=599
66. 281–20=_____
Solution(66):
80-20=60
201+60=261
67. 758–40=_____
Solution(67):
50-40=10
708+10=718
68. 495–70=_____
Solution(68):
90-70=20
405+20=425
69. 164–80=_____
Solution(69):
164-100=64
64+20=84
or,
100-80=20
64+20=84
70. 626–60=_____
Solution(70):
100-60=40
526+40=566
or,
626-100=526
526+40=566
71. 758–300=_____
Solution(71):
700-300=400
400+58=458
72. 834–600=_____
Solution(72):
800-600=200
200+34=234
73. 905–800=_____
Solution(73):
900-800=100
100+5=105
74. 631–500=_____
Solution(74):
600-500=100
100+31=131
75. 978–900=_____
Solution(75):
900-900=0
78+0=78
76. 505–100=_____
Solution(76):
500-100=400
400+5=405
77. 784–400=_____
Solution(77):
700-400=300
300+84=384
78. 435–200=_____
Solution(78):
400-200=200
200+35=235
79. 876–700=_____
Solution(79):
800-700=100
100+76=176
80. 980–800=_____
Solution(80):
900-800=100
100+80=180
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