Question: 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?
Options:
(A) 17 1/9
(B) 17 1/0
(C) 17 1/7
(D) 17 1/2
(E) 17 1/3 Random thoughts:
1/20 + 1/30 - 1/40 = 7/120
120/7 = 17 1/7 The final answer: (C).