Exams › IBPS PO › Quantitative Aptitude

Three pipes A, B and C are opened to fill a tank. B and C can fill the tank alone in 18 hours and 12 hours respectively, and pipe A can empty it in 15 hours. If after 5 hours pipe A is closed, then in how many hours will the remaining tank be filled?

1. None of these
2. 2 hours 50 min.
3. 4 hours 36 min.
4. 1 hours 15 min.

Correct answer: 4 hours 36 min.

Solution

B fills at \(1/18\) tank/hour, C at \(1/12\), and A empties at \(1/15\). So with all three open, net rate is \(1/18+1/12-1/15=13/180\) tank/hour, and in 5 hours they fill \(65/180=13/36\) of the tank. Remaining work is \(23/36\); after A is closed, B and C together fill at \(1/18+1/12=5/36\) tank/hour, so time needed is \((23/36)/(5/36)=23/5=4.6\) hours = 4 hours 36 minutes.

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