Multiple choice

Three pipes fill a tank in 4 hours. If two of them take 8 and 12 hours each to fill the tank, how many hours will the third pipe take to fill the tank?

  1. 6 hours

  2. 15 hours

  3. 18 hours

  4. 24 hours

Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Let the total work be 24 units. Pipe A = 24/8 = 3 units/hr, Pipe B = 24/12 = 2 units/hr. Combined rate of three pipes = 24/4 = 6 units/hr. Third pipe rate = 6 - 3 - 2 = 1 unit/hr. Time = 24/1 = 24 hours.

AI explanation

Let the unknown time for the third pipe be x hours. Using the combined work formula, 1/4 = 1/8 + 1/12 + 1/x. Finding a common denominator for the known fractions gives 1/4 = 3/24 + 2/24 + 1/x, which simplifies to 1/4 = 5/24 + 1/x. Subtracting 5/24 from both sides leaves 1/x = 6/24 - 5/24 = 1/24. Taking the reciprocal, the third pipe will take 24 hours to fill the tank.