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    Question

    Pipe ‘A’ takes 50 hours to fill a tank. Pipe

    ‘B’ takes 100 hours to empty the same tank. If they are opened together, how long will it take to fill 80% of the tank?
    A 82 hours Correct Answer Incorrect Answer
    B 80 hours Correct Answer Incorrect Answer
    C 72 hours Correct Answer Incorrect Answer
    D 78 hours Correct Answer Incorrect Answer

    Solution

    Let the capacity of the tank be 200 litres. So, efficiency of pipe ‘A’ = 200 ÷ A = 4 litres/hour And efficiency of pipe ‘B’ = 200 ÷ B = 2 litres/hour So, combined efficiency of both pipes = 4 – 2 = 2 litres/hour So, time taken to fill 80% of the tank = (200 × 0.80) ÷ 2 = 160 ÷ 2 = 80 hours

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