Question

Pipe X can fill a tank in 12 hours and pipe Y can empty the tank in 18 hours. Both pipes are opened at 8 am and after some time Y is closed, and the tank is full at 10 pm on the same day. At what time was pipe Y closed?

Accepted Answer

C. Let total capacity of the tank = $	ext{LCM}(12, 18) = 36$ units. Efficiency of X = +3 units/hr, efficiency of Y = -2 units/hr. From 8 am to 10 pm is 14 hours. Pipe X was open for all 14 hours, filling $14 	imes 3 = 42$ units. Since capacity is 36 units, Y must have emptied $42 - 36 = 6$ units. Time Y was open = $\frac{6}{2} = 3$ hours. So pipe Y was closed 3 hours after 8 am, which is at 11 am.

Pipes and Cisterns Closing Time

Explanation

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