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    Question

    Ā Pipe A can fill a tank in 12 hours, pipe B in 15

    hours, and an outlet pipe C can empty a full tank in 10 hours. All three are opened together. After 3 hours, pipe C is closed. How much more time will A and B take to fill the tank completely?
    A 15/5 hrs Correct Answer Incorrect Answer
    B 17/3 hrs Correct Answer Incorrect Answer
    C 21/8 hrs Correct Answer Incorrect Answer
    D 33/5 hrs Correct Answer Incorrect Answer

    Solution

    Rates in tank/day (take ā€œunit tankā€): A’s rate = 1/12 B’s rate = 1/15 C’s rate = āˆ’1/10 (emptying) Net rate (A + B + C) = 1/12 + 1/15 āˆ’ 1/10 LCM of 12, 15, 10 is 60: = (5 + 4 āˆ’ 6) / 60 = 3/60 = 1/20 tank/hour Work done in first 3 hours = 3 Ɨ (1/20) = 3/20 of tank. Remaining = 1 āˆ’ 3/20 = 17/20 of tank. After C is closed, only A and B work: (A + B) rate = 1/12 + 1/15 = (5 + 4) / 60 = 9/60 = 3/20 tank/hour Time to finish remaining work = (17/20) Ć· (3/20) = 17/3 hours.

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