It takes 35 identical printing presses 15 hours to print 500,000 papers. How many hours would it take 25 of these printing presses to print 500,000 papers?  Select.
A) 18
B) 20
C) 21
D) 24
E) 25
35 printing presses can do 1/15 of the job each hour.   25 printing presses can do 5/7*1/15 = 1/21 of the job each hour.   The answer is 21.
<result>C</result>
It takes 35 identical printing presses 15 hours to print 500,000 papers. How many hours would it take 25 of these printing presses to print 500,000 papers?