Multiple choice

One pipe can fill a water tank in 40 minutes and another pipe can make the filled tank empty in 60 minutes. If both the pipes are open, in how many hours will the empty tank be filled?

  1. 2 hours

  2. 2.5 hours

  3. 3 hours

  4. 3.5 hours
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

First pipe fills in 40 minutes (rate = +1/40 tank per minute). Second pipe empties in 60 minutes (rate = -1/60 tank per minute). Combined rate = 1/40 - 1/60 = (3-2)/120 = 1/120 tank per minute. Time to fill = 120 minutes = 2 hours.

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