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    Question

    Pipe ‘A’ can fill 25% of a tank in 6 hours

    while pipe ‘B’ takes 40 hours to empty fully filled tank. Pipe ‘A’ is opened alone when the tank was empty. After 12 hours, pipe ‘B’ is also opened. In how much time the tank will get filled this way?
    A 24 hours Correct Answer Incorrect Answer
    B 14 hours Correct Answer Incorrect Answer
    C 4 hours Correct Answer Incorrect Answer
    D 42 hours Correct Answer Incorrect Answer
    E None of these Correct Answer Incorrect Answer

    Solution

    Time taken by pipe ‘A’ to fill the tank = 6/0.25 = 24 hours
    Let, the total capacity of the tank be 120 units
    Efficiency of pipe ‘A’ = 120/24 = 5 units/hour
    Efficiency of pipe ‘B’ = 120/40 = 3 units/hour
    Tank filled in 12 hours = 12 × 3 = 36 units
    Time taken by pipe ‘A’ and pipe ‘B’ to fill the remaining tank = (120 – 36)/(5 – 3) = 30 hours
    Therefore, total time taken = 12 + 30 = 42 hours

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