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?
Choose one: A) 18 B) 20 C) 21 D) 24 E) 25

Rationale: "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 c ."
Answer: C