MENTAL ARITHMETIC: SUBTRACTION

PROBLEMS 3

Completing 100

19.  If an item costs 59¢, and you give a dollar bill, how much is your
18.  change?  41¢

20.  What number must you add to each of these to make 100?  
20.  (That number is called the complement to 100.)

86 +  14   74 +  26   42 +  58   58 +  42   63 +  37  
 
31 +  69   79 +  21   46 +  54   84 +  16   51 +  49  

21.  How much must you add to each of these to make $10.00?

$5.60 +  $4.40   $5.15 +  $4.85   $6.37 +  $3.63   $2.28 +  $7.72  
 
$1.24 +  $8.76   $7.56 +  $2.44   $3.44 +  $6.56   $8.53 +  $1.47  

22.  44% voted No.  What percent voted Yes?  56%

23.  The price of a jacket was reduced by 35%.  What percent of the
23.  original price do you pay?  65%

24.  Use the complement to 100 of the smaller number to calculate
24.  each column mentally.

109 − 75   25 + 9 = 34

103 − 85 = 18          132 − 78 = 54

115 − 92 = 23          106 − 47 = 59

124 − 88 = 36          111 − 35 = 76

130 − 79 = 51          124 − 75 = 49

115 − 86 = 29          152 − 94 = 58

25.  William Shakespeare was born in 1564 and died in 1616.  How old
25.  was he when he died?  52

26.  How long ago was the Stock Market crash of 1929?

Start at 1929, and count to the present year.

27.  How many years have passed since the discovery of America
26. (1492)?

Start at 1492, and count to the present year.

28.  How long ago did the Pilgrims land at Plymouth Rock (1620)?

Start at 1620, and count to the present year.

29.  How many years have passed since the signing of the Declaration of
27.  Independence (1776)?

Start at 1776, and count to the present year.

30.  How long ago was the Battle of Hastings (1066)?

Start at 1066, and count to the present year.

Rounding off

31.  Consider  231 − 95.

31.  a)  If you add 5 to both numbers, how will the difference change ?

The difference will not change.

31.  b)  Calculate the difference mentally by rounding off.

236 − 100 = 136

32.  How does compensating in subtraction differ from compensating in
32.  addition?

In subtraction, if we add to one number, we must add it to the other number. But in addition, if we add to one number, we must subtract it from another or from the answer.

33.  Calculate each column mentally by rounding off the smaller number.

a)   b)
72 − 19 = 53   82 − 29 = 53
64 − 29 = 35   55 − 17 = 38
91 − 48 = 43   94 − 69 = 25
43 − 18 = 25   73 − 25 = 48
95 − 27 = 68   93 − 56 = 37
c)   d)
246 − 99 = 147   732 − 199 = 533
325 − 98 = 227   843 − 298 = 545
471 − 95 = 376   925 − 496 = 429
520 − 97 = 423   1811 − 189 = 1622
612 − 88 = 524   4733 − 485 = 4248

34.  The cents in the smaller number are less.  Calculate mentally by
31.  rounding off the cents in the smaller number.  Subtract the cents
31.  first.

4.88 − 1.59 = 3.29   4.48 − 2.19 = 2.29
 
8.95 − 3.49 = 5.46   7.65 − 5.36 = 2.29
 
3.70 − 1.28 = 2.42   8.90 − 7.47 = 1.43
 
9.85 − 4.49 = 5.36   12.52 − 3.35 = 9.17
 
5.91 − 2.45 = 3.46   11.83 − 5.56 = 6.27

35.  The cents in the smaller number are more.  Calculate mentally
35.  by rounding off the smaller number to the next whole number.

18.00 − 4.85 = 13.15   7.10 − 2.90 = 4.20
 
29.00 − 8.48 = 20.52   7.20 − 3.60 = 3.60
 
32.00 − 5.57 = 26.43   8.15 − 1.88 = 6.27
 
43.01 − 9.78 = 33.23   6.34 − 2.76 = 3.58
 
51.03 − 7.65 = 43.38   9.16 − 1.89 = 7.27

9.20 − 3.72 = 5.48

7.12 − 2.49 = 4.63

8.23 − 6.84 = 1.39

9.12 − 4.25 = 4.87

5.21 − 3.75 = 1.46


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