Pipes A and B can fill a tank in 5 and 6 hours respectively. Pipe C can empty it in 12 hours. If all three pipes are opened together, then the tank will be filled in:

1 13/17 hours
2 8/11 hours
3 9/17 hours
4 1/2 hours

Correct answer: 3 9/17 hours

Solution

A fills \(1/5\) tank/hour, B fills \(1/6\) tank/hour, and C empties \(1/12\) tank/hour. Net rate = \(1/5 + 1/6 - 1/12 = 17/60\) tank/hour, so time taken = \(60/17 = 3\frac{9}{17}\) hours.

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