Give stream of consciousness and then the final answer. Two pipes A and B can fill a cistern in 20 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) 19 1/7 min\n(B) 15 1/7 min\n(C) 17 1/7 min\n(D) 7 1/7 min\n(E) 7 2/7 min
User
1/20 + 1/30 - 1/40 = 7/120\n120/7 = 17 1/7
The final answer: (C).
Assistant