Question
Pipe A and Pipe B could fill a tank together in 18
hours and 12 hours respectively, while pipe C could empty it in 9 hours. If both inlet pipes are opened together for first 5 hours, then pipe C replaced them. Find the time taken by pipe C to empty the tank filled by pipe A and pipe B together.Solution
Let the capacity of the tank be 36 units. (LCM of 18, 12 and 9)
Efficiency of pipe A = 36/18 = 2 units/hr
Efficiency of pipe B = 36/12 = 3 units/hr
Efficiency of pipe C = 36/9 = –4 units/hr ATQ,
Tank filled by pipe A and B together in 5 hours = (2 + 3) × 5 = 25 units
Required time = 25/4 = 6 (1/4) hr = 6 hr 15 min
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