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    Question

    A pipe can fill a tank in 12 hours, while a second pipe

    can empty it in 18 hours. If both pipes are opened at the same time, how long will it take to fill the tank?
    A 36 hours Correct Answer Incorrect Answer
    B 40 hours Correct Answer Incorrect Answer
    C 45 hours Correct Answer Incorrect Answer
    D 50 hours Correct Answer Incorrect Answer

    Solution

    Rate of filling of the first pipe = 1/12 tank per hour. Rate of emptying of the second pipe = 1/18 tank per hour. Net rate = 1/12 - 1/18 = (3 - 2) / 36 = 1/36 tank per hour. Time to fill the tank = 1 / (1/36) = 36 hours. Correct answer: a. 36 hours

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