If 20 typists can type 42 letters in 20 minutes, then how many letters will 30 typists working at the same rate complete in 1 hour?
Choose the most appropriate option:
A) 189	B) 72	C) 144	D) 216	E) 400

Rationale: "20 typists can type 42 letters , so 30 typists can type = 42 * 30 / 20 42 * 30 / 20 letters can be typed in 20 mins . in 60 mins typist can type = 42 * 30 * 60 / 20 * 20 = 189 a is the answer"
Answer: A