Multiple choice

Two pipes can fill a tank in 15 hours and 12 hours and a third pipe can empty full tank in 6 hours. First two pipes opened for 5 hours and then third pipe is also opened. In which time the tank will emptied?

  1. 30 hours

  2. 35 hours

  3. 40 hours

  4. 45 hours
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Pipes 1 and 2 fill at rates of 1/15 and 1/12 per hour respectively. In 5 hours, they fill: (1/15 + 1/12) × 5 = (9/60) × 5 = 45/60 = 3/4 of the tank. When all three pipes open, the net rate becomes: 1/15 + 1/12 - 1/6 = (4+5-10)/60 = -1/60 per hour (emptying). To empty 3/4 tank at this rate: (3/4) ÷ (1/60) = 45 hours.

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