Multiple choice

Two pipes X and Y can fill a cistern in 6 hours and 10 hours respectively. Pipe Z can empty the tank in 4 hours. If all the three pipes are open then in how many hours the cistern will be full?

  1. 30

  2. 45

  3. 50

  4. 60
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

X fills 1/6 per hour, Y fills 1/10 per hour, Z empties 1/4 per hour. Combined rate = 1/6 + 1/10 - 1/4 = (10+6-15)/60 = 1/60. Time = 1/(1/60) = 60 hours. The negative sign for emptying pipe is crucial.

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