Practice mode Pipes and Cistern

An inlet fills a tank in 3 hours and an outlet empties it in 4 hours. Starting with the inlet, they are opened alternately for one hour each. After how many hours will the empty tank first be full?

Correct answer D. 17 hours

Explanation

Each 2-hour cycle adds 1/3 − 1/4 = 1/12. After 8 cycles (16 hours) the tank is 2/3 full, and the inlet then fills the last 1/3 in the 17th hour, so 17 hours.

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Answer checked 11 October 2026 How we write and check questions

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An inlet fills a tank in 3 hours and an outlet empties it in 4 hours. Starting with the inlet, they are opened alternately for one hour each. After how many hours will the empty tank first be full?

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An inlet fills a tank in 3 hours and an outlet empties it in 4 hours. Starting with the inlet, they are opened alternately for one hour each. After how many hours will the empty tank first be full?

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