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?  Choose one
A) 63	B) 72	C) 144	D) 180	E) 400
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   180 is the answer
<result>D</result>
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?