If 20 typists can type 40 letters in 20 minutes, then how many letters will 30 typists working at the same rate complete in 1 hour? Pick one:	A) 63	B) 72	C) 144	D) 180	E) 400

Rationale: "20 typists can type 40 letters , so 30 typists can type = 40 * 30 / 20 40 * 30 / 20 letters can be typed in 20 mins . in 60 mins typist can type = 40 * 30 * 60 / 20 * 20 = 180 d is the answer"
Answer: D