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    Question

    Pipe P can fill a tank in 5 hours, but pipe Q can empty

    it completely in 20 hours. If both are opened together, find the time needed to fill the tank.
    A 25 hrs Correct Answer Incorrect Answer
    B 14 hrs Correct Answer Incorrect Answer
    C 4.55 hrs Correct Answer Incorrect Answer
    D 6.66 hrs Correct Answer Incorrect Answer

    Solution

    Let tank capacity = 20 litres {LCM (5 and 20)} Efficiency of filling pipe P = (20/5) = 4 units/hour Efficiency of emptying pipe Q = (20/20) = 1 unit/hour Net working efficiency = 4 тАУ 1 = 3 units/hour Required time = (20/3) = 6.66 hours

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