Multiple choice

A pump can fill a cistern in 40 minutes and another pump can empty the filled up cistern in 1 hour. By mistake, without closing the second pump, the first pump is opened. The time in which the empty cistern will be filled up is-

  1. 3 hours

  2. 4 hours

  3. 1 hour

  4. 2 hours

  5. None of these
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

First pump fills in 40 minutes, rate = 1/40 per minute. Second pump empties in 60 minutes, rate = -1/60 per minute. Net rate = 1/40 - 1/60 = (3-2)/120 = 1/120 per minute. Time to fill = 120 minutes = 2 hours. Option D matches.

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