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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?

/ Mathematics Mcqs, Pipes and Cisterns / By
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?

A. 19 1/7 min
B. 15 1/7 min
C. 17 1/7 min
D. 7 1/7 min
Explanation:
1/20 + 1/30 – 1/40 = 7/120
120/7 = 17 1/7