Multiple choice

Two pipes P and Q would fill a tank in 12 hours and 16 hours respectively. If both pipes are opened together, find when the first pipe must be turned off so that the tank may be just filled in 8 hours.

  1. 4 hours

  2. 5 hours

  3. 6 hours

  4. 3 hours
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Pipe P fills 1/12 per hour, Q fills 1/16 per hour. Let P run for t hours, then Q runs for full 8 hours. Work by P + Work by Q = 1 full tank. (t/12) + (8/16) = 1. t/12 + 1/2 = 1. t/12 = 1/2. t = 6 hours. P must be turned off after 6 hours. Option C is correct.

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