MENTAL ARITHMETIC: SUBTRACTION

PROBLEMS 2

Finding the difference by the ending

11.  a)  5 + 8 ends in 3.  b)  9 + 5 ends in 4.  c)  6 + 9 ends in 5.

13.  d)  7 + 4 ends in 1.  e)  8 + 7 ends in 5.  f)  7 + 6 ends in 3.

12.   12 − 7 = 5

14.  Whenever you take a 7 from a 2, the answer will always end in 5

14.  And the answer falls in the previous decade.

32 − 7 = 25

42 − 7 = 35

62 − 7 = 55

92 − 7 = 85

102 − 7 = 95  

13.  The difference in the previous decade.  Calculate mentally by
15.  deciding on the ending.

31 − 8 = 23  63 − 7 = 56  62 − 8 = 54

43 − 9 = 34  84 − 8 = 76  74 − 7 = 67

56 − 9 = 47  22 − 9  = 13  81 − 6 = 75

75 − 8 = 67  45 − 9 = 36  23 − 8 = 15

21 − 8 = 13  31 − 9 = 22  52 − 8 = 44

The difference between two-digit numbers

14.  What number must you add?  First complete a 10.  Illustrate that in
11.   writing.

57 + ? = 61.   3 + 1 = 4. (57 + 3 is 60; plus 1.)

86 + ? = 92.   4 + 2 = 6.

15.  What number must you add to the smaller number?  
11.  First, complete a 10.  Or decide on the ending.

31 − 27 = 4		85 − 77 = 8		21 − 18 = 3
43 − 38 = 5		92 − 83 = 9		83 − 76 = 7
54 − 47 = 7		27 − 18 = 9		42 − 38 = 4
62 − 56 = 6		56 − 47 = 9		57 − 49 = 8
72 − 69 = 3		82 − 74 = 8		65 − 56 = 9

16.  What number must you add?

25 + 33 = 58

38 + 41 = 79

32 + 65 = 97

17.  Add to complete a multiple of 10.  Add to get into the previous
17.  decade.

25 + 35 = 60.		38 + 42 = 80.		32 + 58 = 90
37 + 23 = 60.		26 + 54 = 80.		48 + 42 = 90.

18.  The ones of the smaller number greater.  Decide the ending.
18.  Then add to get to the previous decade.

34 + 27 = 61

35 + 37 = 72  33 + 48 = 81

54 + 28 = 82  44 + 28 = 72

46 + 45 = 91  22 + 49 = 71

23 + 39 = 62  45 + 38 = 83

54 + 39 = 93  64 + 27 = 91

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