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    Question

    Pipe Y is 25% less efficient than

    pipe X. Together, pipes X and Y can fill a tank in 15 minutes. Pipe Z alone can empty the tank in 70 minutes. How long will it take for pipes X and Z working together to fill the empty tank?
    A 24 minutes Correct Answer Incorrect Answer
    B 36 minutes Correct Answer Incorrect Answer
    C 42 minutes Correct Answer Incorrect Answer
    D 18 minutes Correct Answer Incorrect Answer

    Solution

    ATQ,

    Let the capacity of the tank = L.C.M of 15 and 70 = 210 units Then, combined eiciency of pipes ‘X’ and ‘Y’ = 210 ÷ 15 = 14 units/minute Efficiency of pipe ‘X’ alone = (14 ÷ 1.75) × 1 = 8 units/minute Efficiency of pipe ‘Z’ alone = 210 ÷ 70 = 3 units/minute (outlet) So, combined efficiency of pipes ‘X’ and ‘Z’ = 8 – 3 = 5 units/minute So, time taken by pipes ‘X’ and ‘Z’ together to fill the tank = 210 ÷ 5 = 42 minutes 

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