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    Question

    Pipe A and Pipe B could fill a tank together in 25 hours

    and 20 hours respectively, while pipe C could empty it in 10 hours. If both inlet pipes are opened together for first 2 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 1 hr 36 min Correct Answer Incorrect Answer
    B 1 hr 42 min Correct Answer Incorrect Answer
    C 1 hr 48 min Correct Answer Incorrect Answer
    D 2 hr Correct Answer Incorrect Answer
    E 1 hr 30 min Correct Answer Incorrect Answer

    Solution

    Let the capacity of the tank be 100 units. (LCM of 25, 20 and 10)
    Efficiency of pipe A = 100/25 = 4 units/hr
    Efficiency of pipe B = 100/20 = 5 units/hr
    Efficiency of pipe C = 100/10 = –10 units/hr ATQ,
    Tank filled by pipe A and B together in 2 hours = (4 + 5) × 2 = 18 units
    Required time = 18/10 = 1.8 hr = 1 hr 48 min

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