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    Question

    Three pipes A, B and C can fill/empty a tank. A can fill

    it in 12 hours, B in 15 hours, and C can empty a full tank in 20 hours. All three pipes are opened together. After some time, pipe C is closed and A and B together fill the remaining tank in 5 hours. If the tank is exactly full at the end, for how many hours was pipe C kept open?
    A 4.5 hrs Correct Answer Incorrect Answer
    B 2.5 hrs Correct Answer Incorrect Answer
    C 8.4 hrs Correct Answer Incorrect Answer
    D 9.4 hrs Correct Answer Incorrect Answer

    Solution

    ATQ, Filling rates: A = 1/12, B = 1/15, C (emptying) = −1/20 (per hour). A + B = 1/12 + 1/15 = (5 + 4)/60 = 9/60 = 3/20. A + B + C = 3/20 − 1/20 = 2/20 = 1/10. Let C be open for x hours (all three working). Volume filled in this time = x × (1/10) = x/10. After C is closed, A + B work for 5 hours: Volume filled = 5 × (3/20) = 15/20 = 3/4. Total volume = 1 ⇒ x/10 + 3/4 = 1  ⇒ x/10 = 1 − 3/4 = 1/4  ⇒ x = 2.5 hours.

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