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    Question

    Pipe ‘M’ and pipe ‘N’ can fill a tank alone in

    20 hours and 40 hours, respectively. If both pipes are opened together and pipe ‘M’ is closed after 5 hours, then find the time taken by pipe ‘N’ to fill the remaining portion alone.
    A 15 hours Correct Answer Incorrect Answer
    B 10 hours Correct Answer Incorrect Answer
    C 25 hours Correct Answer Incorrect Answer
    D 20 hours Correct Answer Incorrect Answer

    Solution

    Let the capacity of the tank be 40 units (LCM of 20 and 40) Efficiency of pipe ‘M’ = 40 ÷ 20 = 2 unit/hour
    Efficiency of pipe ‘N’ = 40 ÷ 40 = 1 unit/hour Let pipe ‘N’ fill the remaining portion in x hours. ATQ, 5 × (2 + 1) + 1x = 40 Or, 15 + x = 40 Or, x = 25 Therefore, required time taken = 25 hours

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