SSC & BANK EXAMS: QUANTITATIVE APTITUDE · TIME & WORK

Pipes and cisterns

Filling pipes add to the rate, emptying pipes subtract. Take tank capacity = LCM of the times.

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Worked examples

Example 1 · level 1

Pipes A and B can fill a swimming pool in 28 hours and 21 hours respectively. If both are opened together, in how many hours will the tank be full?

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Capacity = 84 units.
Net rate = 3 + 4 = 7 units/h.
Time = 84 ÷ 7 = 12 hours.

Answer: 12 hours

Example 2 · level 2

Pipes A and B can fill a cistern in 2 hours and 1 hours respectively, while pipe C can empty the full tank in 2 hours. If all three are opened together on an empty tank, how many hours will it take to fill it?

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Capacity = 2 units.
Net rate = 1 + 2 − 1 = 2 units/h.
Time = 2 ÷ 2 = 1 hours.

Answer: 1 hours

Example 3 · level 3

Pipes A and B can fill a swimming pool in 3 hours and 5 hours respectively, while pipe C can empty the full tank in 5 hours. If all three are opened together on an empty tank, how many hours will it take to fill it?

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Capacity = 15 units.
Net rate = 5 + 3 − 3 = 5 units/h.
Time = 15 ÷ 5 = 3 hours.

Answer: 3 hours