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    Question

    Pipe A and Pipe B could fill a tank together in 30 hours

    and 20 hours respectively, while pipe C could empty it in 12 hours. If both inlet pipes are opened together for first 6 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 5 hr 30 min Correct Answer Incorrect Answer
    B 6 hr 30 min Correct Answer Incorrect Answer
    C 4 hr 45 min Correct Answer Incorrect Answer
    D 6 hr Correct Answer Incorrect Answer

    Solution

    Let the capacity of the tank be 60 units. (LCM of 30, 20 and 12)
    Efficiency of pipe A = 60/30 = 2 units/hr
    Efficiency of pipe B = 60/20 = 3 units/hr
    Efficiency of pipe C = 60/12 = –5 units/hr (‘−’ sign, since it’s an outlet pipe) ATQ,
    Tank filled by pipe A and B together in 6 hours = (2 + 3) × 6 = 30 units
    Required time = 30/5 = 6 hr

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