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?
Answers
A) 17 1/8 min  B) 17 2/7 min  C) 17 1/7 min  D) 17 1/6 min  E) 17 3/7 min
Explanation:   1/20 + 1/30 - 1/40   = 7/120   120/7   = 17 1/7   Answer:17 1/7 min
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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?