Multiple choice

Three pipes can fill a tank in 15, 20, and 25 hours respectively. Two other pipes can empty it in 30 and 40 hours respectively. If all the pipes are opened together, in how many hours will the empty tank be filled (approximately)?

  1. 9.8 hours

  2. 10.2 hours

  3. 10.5 hours

  4. 11.4 hours
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Net rate = 1/15 + 1/20 + 1/25 - 1/30 - 1/40. Common denominator = 600. Rate = (40 + 30 + 24 - 20 - 15) / 600 = 59 / 600. Time = 600 / 59 approx 10.17 hours.

AI explanation

Using the least common multiple method, assume the total capacity of the tank is 600 units. The filling pipes work at rates of 40, 30, and 24 units per hour, while the emptying pipes work at rates of 20 and 15 units per hour. The net rate is 40 + 30 + 24 - 20 - 15 = 59 units per hour. The total time to fill the tank is 600 / 59, which is approximately 10.2 hours.

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