MENTAL ARITHMETIC: MULTIPLICATION

PROBLEMS (2)

The distributive property

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13.  Calculate in writing by distributing the multiplier to each unit of the
12.  multiplicand.  Treat problems with decimal points as dollars and
12.  cents.

13.  a)  3 × 42The Distributive Law

13.  b)  5 × 34 = 5 × 30 + 5 × 4 = 150 + 20 = 170

13.  c)  6 × 32 = 6 × 30 + 6 × 2 =180 + 12 = 192

13.  d)  7 × 56 = 350 + 42 = 392

13.  e)  8 × 97 = 720 + 56 = 776

13.  f)  2 × 346 = 600 + 80 + 12 = 692

13.  g)  3 × 475 = 1200 + 210 + 15 = 1425

13.  h)  7 × 908 = 6300 + 56 = 6,356

13.  i)  9 × 4,005 = 36,000 + 45 = 36,045

13.  j)  6 × 7.30 = 42 + 1.80 = 43.80

13.  k)  9 × 4.50 = 36 + 4.50 = 40.50

14.  Whatever you wrote in the previous problem, do mentally here.

6 × 13 = 78   7 × 55 = 385   3 × 96 = 288
 
3 × 72 = 216   9 × 72 = 648   5 × 68 = 340
 
8 × 34 =  272   8 × 36 =  288   6 × 36 =  216
 
4 × 49 =  196   6 × 89 =  534   4 × 38 =  152
 
9 × 52 =  468   8 × 28 =  224   9 × 36 =  324

15.  Calculate each column mentally.

30 × 27 = 810   60 × 620 = 37,200
 
40 × 34 = 1360   70 × 770 = 53,900
 
50 × 59 = 2950   80 × 980 = 78,400

16.  You have 47 twenty-dollar bills.  Calculate mentally how much
16.  money you have.

2 × 47 = 80 + 14 = 94. Therefore 20 × 47 = $940.

17.  Multiply each column mentally.  These are dollars and cents.

4 × .25 = 1.00   6 × 3.25 = 19.50   7 × 9.50 = 66.50
 
5 × .25 = 1.25   5 × 6.25 = 31.25   3 × 4.80 = 14.40
 
6 × .25 = 1.50   6 × 9.25 = 55.50   9 × 5.20 = 46.80
 
7 × .25 = 1.75   7 × 8.25 = 57.75   4 × 6.30 = 25.20
 
8 × .25 = 2.00   8 × 4.25 = 34.00   7 × 7.40 = 51.80
 
9 × .25 = 2.25   9 × 5.25 = 47.25   8 × 3.90 = 31.20

18.  Multiply each column mentally by distributing the multiplier to the
18.  hundreds, then the tens, then the ones..

2 × 423 = 846   7 × 309 = 2163
 
3 × 624 = 1872   8 × 602 = 4816
 
4 × 574 = 2296   9 × 907 = 8163
 
5 × 357 = 1785   5 × 2009 = 10,045

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