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    Question

    Pipe A and Pipe B could fill a tank together in 40 hours

    and 16 hours respectively, while pipe C could empty it in 20 hours. If both inlet pipes are opened together for first 4 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.
    A 6 hr 45 min Correct Answer Incorrect Answer
    B 7 hr Correct Answer Incorrect Answer
    C 6 hr 30 min Correct Answer Incorrect Answer
    D 5 hr 40 min Correct Answer Incorrect Answer
    E 7 hr 15 min Correct Answer Incorrect Answer

    Solution

    Let the capacity of the tank be 80 units. (LCM of 40, 16 and 20)
    Efficiency of pipe A = 80/40 = 2 units/hr
    Efficiency of pipe B = 80/16 = 5 units/hr
    Efficiency of pipe C = 80/20 = –4 units/hr ATQ,
    Tank filled by pipe A and B together in 4 hours = (2 + 5) × 4 = 28 units
    Required time = 28/4 = 7 hr

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