Give a quick stream of consciousness before answering the following question. Two pipes A and B can fill a cistern in 15 and 30 minutes respectively, and a third pipe C can empty it in 40 minutes. How long will it take to fill the cistern if all the three are opened at the same time?\nOptions:\n(A) 17 1/4 min\n(B) 28 1/7 min\n(C) 17 1/7 min\n(D) 13 3/7 min\n(E) 17 5/7 min
User
1/15 + 1/30 - 1/40 = 9/120\n120/9 = 13 3/7
The answer: (D).
Assistant