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    Question

    Pipe P can fill a cistern in 12 hours and pipe Q in 20

    hours. If both the pipes are opened in an empty cistern, how much time will they take to fill it?
    A 10 hours Correct Answer Incorrect Answer
    B 7.5 hours Correct Answer Incorrect Answer
    C 8 hours Correct Answer Incorrect Answer
    D 12 hours Correct Answer Incorrect Answer

    Solution

    Let the capacity of tank be 60 litres (LCM of 12 and 20)
    Efficiency of 'P' = (60 / 12) = 5 litres/hour
    Efficiency of 'Q' = (60 / 20) = 3 litres/hour
    Required time = 60 ÷ (5 + 3) = 7.5 hours

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