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Question 1:
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
Feedback 1/20 + 1/30 - 1/40 = 7/120
120/7 = 17 1/7.
D. 7 1/7 min

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