Pipes & Cisterns · Question 27 · Hard
Pipes & Cisterns question 27
An inlet pipe can fill a tank in 4 hours and an outlet pipe can empty the full tank in 6 hours. Starting with an empty tank, they are opened alternately for one hour each, beginning with the inlet. After how many hours will the tank be full?
- 19 hours
- 18 hours
- 24 hours
- 20 hours
Answer and explanation
Answer: A. 19 hours
Explanation: Each 2-hour cycle adds 1/4 − 1/6 = 1/12. After 9 cycles (18 hours) the tank is 9/12 = 3/4 full, and the inlet's next hour adds 1/4, filling it at 19 hours. (After 8 cycles the level is only 8/12, and one more inlet hour gives 11/12.)
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