MENTAL ARITHMETIC: SUBTRACTION

PROBLEMS

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11.  Explain why  41 − 35 = 6.

Because 35 plus 6 is 41.

12.  Practice the following.  Do not count on your fingers Exclamation!

2 + 7 = 9.   1 + 5 = 6.   2 + 3 = 5.   2 + 6 = 8.
 
3 + 4 = 7.   5 + 4 = 9.   3 + 2 = 5.   1 + 7 = 8.
 
5 + 3 = 8.   1 + 4 = 5.   5 + 2 = 7.   6 + 3 = 9.
 
4 + 3 = 7.   4 + 5 = 9.   2 + 2 = 4.   3 + 3 = 6.
 
1 + 8 = 9.   1 + 5 = 6.   1 + 2 = 3.   3 + 5 = 8.

13.  Calculate each column mentally by adding to the smaller number.

8 − 5 = 3   90 − 60 = 30   95 − 40 = 55   43 − 20 = 23
 
80 − 50 = 30   80 − 30 = 50   63 − 10 = 53   78 − 20 = 58
 
825 − 500 = 325   84 − 30 = 54   62 − 30 = 32   94 − 30 = 64
 
    95 − 70 = 25   72 − 50 = 22
 
    48 − 30 = 18   81 − 40 = 41
 
    99 − 20 = 79   66 − 50 = 16

14.  What number must you add?

8 + 6 = 14.   8 + 5 = 13.   7 + 9 = 16.   8 + 3 = 11.
 
2 + 9 = 11.   4 + 8 = 12.   9 + 8 = 17.   7 + 6 = 13.
 
5 + 7 = 12.   9 + 7 = 16.   6 + 5 = 11.   5 + 6 = 11.
 
6 + 9 = 15.   5 + 9 = 14.   5 + 8 = 13.   8 + 9 = 17.
 
9 + 4 = 13.   6 + 6 = 12.   8 + 7 = 15.   6 + 8 = 14.
 
7 + 5 = 12.   4 + 9 = 13.   9 + 3 = 12.   4 + 7 = 11.
 
3 + 8 = 11.   7 + 8 = 15.   9 + 2 = 11.   8 + 4 = 12.
 
6 + 7 = 13.   7 + 4 = 11.   9 + 5 = 14.   3 + 9 = 12.

15.  What number must you add to the smaller number?

120 − 90 = 30   1900 − 1200 = 700
 
126 − 90 = 36   1800 − 1400 = 400
 
135 − 90 = 45   1842 − 1400 = 442
 
124 − 80 = 44   1986 − 1600 = 386
 
138 − 80 = 58   2004 − 1500 = 504
 
146 − 80 = 66   2110 − 1700 = 410

The complement to 10

    6. 3 + 7 = 10   9 + 1 = 10   4 + 6 = 10   2 + 8 = 10
 
  1 + 9 = 10   8 + 2 = 10   6 + 4 = 10   5 + 5 = 10
 
  7 + 3 = 10

17.  More complements to 10.

30 − 2 = 28   40 − 6 = 34   50 − 3 = 47   60 − 9 = 51
 
40 − 5 = 35   50 − 1 = 49   80 − 4 = 76   90 − 4 = 86
 
50 − 9 = 41   70 − 4 = 66   20 − 3 = 17   50 − 7 = 43
 
20 − 6 = 14   30 − 5 = 25   70 − 8 = 62   20 − 9 = 11

*     *     *

100 − 3 = 97   220 − 1 = 219
 
100 − 7 = 93   350 − 3 = 347
 
110 − 6 = 104   590 − 7 = 583
 
120 − 8 = 112   600 − 5 = 595
 
130 − 5 = 125   700 − 2 = 698

18.  Calculate mentally.

$4.00 − 3¢ = $3.97   $7.00 − 8¢ = $6.92   $8.00 − 9¢ = $7.91
 
$9.00 − 6¢ = $8.94   $1.00 − 4¢ = $.96   $6.00 − 7¢ = $5.93

19.  Subtracting a one-digit number.  Break off from the smaller
19.  number the ones digit of the larger, and subtract.  Then subtract the
19.  rest of the smaller number.

Illustrate that in writing.

61 − 3 = 61 − 1 − 2 = 60 − 2 = 58

93 − 4 = 93 − 3 − 1 = 90 − 1 = 89

10.  Calculate mentally.

22 − 3 = 19   61 − 2 = 59   71 − 4 = 67
 
53 − 4 = 49   83 − 5 = 78   22 − 4 = 18
 
31 − 3 = 28   62 − 5 = 57   91 − 2 = 89
 
21 − 4 = 17   42 − 3 = 39   92 − 4 = 88
 
43 − 5  = 38   72 − 4 = 68   52 − 5 = 47
 
73 − 4 = 69   24 − 5 = 19   44 − 5 = 39

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