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    Question

    Pipe ‘X’ alone can fill a tank in 20 hours. When both

    pipes ‘X’ and ‘Y’ are opened together, they can fill 75% of the tank in 9 hours. How much time will pipe ‘Y’ alone take to fill 80% of the same tank?
    A 24 hrs Correct Answer Incorrect Answer
    B 15 hrs Correct Answer Incorrect Answer
    C 18 hrs Correct Answer Incorrect Answer
    D 12 hrs Correct Answer Incorrect Answer
    E None of these Correct Answer Incorrect Answer

    Solution

    ATQ,
    Time taken by pipes ‘X’ and ‘Y’ together to fill 100% of the tank = 9 ÷ 0.75 = 12 hours
    Let the capacity of the tank be 60 litres. {LCM (20 and 12)}
    Efficiency of pipe ‘X’ = (60/20) = 3 litres/hour
    Efficiency of pipe ‘X’ and ‘Y’ together = (60/12) = 5 litres/hour
    Efficiency of pipe ‘Y’ = 5 – 3 = 2 litres/hour
    Time taken by pipe ‘Y’ to fill 80% of the tank = (60 × 0.8)/2 = 24 hours

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