Multiple choice

Two inlet pipes A and B can fill a tank in 24 hours and 30 hours, respectively. An outlet pipe C can empty the tank in 60 hours. The pipe C is continuously open and the pipes A and B work on alternate hours, starting with A. How much time does it take to fill the tank?

  1. 36 hours

  2. 48 hours

  3. 40 hours

  4. 24 hours

  5. 20 hours
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

A fills 1/24, B fills 1/30, C empties 1/60. In 2 hours (A+B), net work is (1/24 - 1/60) + (1/30 - 1/60) = 1/40 + 1/60 = 5/120 = 1/24. It takes 24 cycles of 2 hours to fill, so 48 hours.

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