Answer:
20. 20 workers
21. 15 hits
Explanation:
20. To complete the job in 9/15 of the time will require 15/9 times as many workers, since the relation is inversely proportional.
15/9·12 = 20
20 workers will be required.
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21. You want x such that ...
(12+x)/(60+x) = 0.360
12 +x = 0.360(60 +x) . . . . multiply by the denominator
0.64x = 21.6 -12 . . . . . . . . subtract 12+0.36x
9.6/0.64 = x = 15 . . . . . . . divide by the coefficient of x
15 consecutive hits are required to raise the batting average.
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This can also be worked by rewriting the rational expression.
1 - 48/(60+x) = 0.360 . . . . rewrite to quotient and remainder
1 - 0.360 = 48/(60 +x) . . . add 48/(60+x) - 0.360
60 +x = 48/0.64 = 75 . . . . multiply by (60+x)/0.64
x = 15 . . . . . . . . . . . . . . . . . subtract 60