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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
- A19 1/7 min
- B15 1/7 min
- C17 1/7 min
- D7 1/7 min
Correct answer: C