Multiple choice

There are two inlet pipes A and B. Pipe A fills a tank alone in 6 hours, work efficiency of B is 25% more than pipe A's work efficiency. While pipe C can empty the tank in 12 hours alone. If all the pipes are opened simultaneously, in how many hours 83.33% of the tank will be filled?

  1. 20/7

  2. 10/7

  3. 9/6

  4. 2/5
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Pipe A fills 1/6 per hour. Pipe B is 25% more efficient, so fills (1.25 × 1/6) = 5/24 per hour. Pipe C empties 1/12 per hour. Combined rate = 1/6 + 5/24 - 1/12 = 4/24 + 5/24 - 2/24 = 7/24 per hour. 83.33% = 5/6. Time = (5/6) ÷ (7/24) = (5/6) × (24/7) = 20/7 hours.

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