It takes 40 identical printing presses 9 hours to print 500,000 papers. How many hours would it take 30 of these printing presses to print 500,000 papers? Answers
A) 9	B) 10	C) 11	D) 12	E) 13

Rationale: "40 printing presses can do 1 / 9 of the job each hour . 30 printing presses can do 3 / 4 * 1 / 9 = 1 / 12 of the job each hour . the answer is d ."
Answer: D