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    Question

    Two pipes can fill a tank in 10 hours and 12 hours,

    respectively, while a third pipe can empty it in 20 hours. If all the three pipes are opened simultaneously, how long will it take to fill the tank?
    A 8 hours Correct Answer Incorrect Answer
    B 7.5 hours Correct Answer Incorrect Answer
    C 6 hours Correct Answer Incorrect Answer
    D 5.5 hours Correct Answer Incorrect Answer

    Solution

    Let the total work be 1 unit. Rate of filling by first pipe = 1/10, Rate of filling by second pipe = 1/12, Rate of emptying by third pipe = 1/20. Net rate of filling = 1/10 + 1/12 - 1/20 = 8/60 = 2/15. Time = 1 / (2/15) = 7.5 hours. Answer: b)

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