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    Question

    Pipe 'A' alone can fill a cistern

    in 20 minutes. If pipes 'A' and 'B' are opened together, it takes 25 minutes to fill the cistern. Find the time taken by pipe 'B' alone to empty 50% of the cistern.
    A 65 min Correct Answer Incorrect Answer
    B 45 min Correct Answer Incorrect Answer
    C 50 min Correct Answer Incorrect Answer
    D 25 min Correct Answer Incorrect Answer

    Solution

    ATQ, Total capacity of the tank = L.C.M of 20 and 25 = 100 units. Efficiency of pipe 'A' alone = 100 ÷ 20 = 5 units/minute. Combined efficiency of pipes 'A' and 'B' = 100 ÷ 25 = 4 units/minute. Efficiency of pipe 'B' alone = 4 - 5 = -1 unit/minute (outlet). Time taken by pipe 'B' alone to empty 50% of the cistern = (100 × 0.5) ÷ 1 = 50 minutes.

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