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    Question

    Three pipes A, B, and C can fill a tank in 15 hours, 20

    hours, and 25 hours respectively. All three pipes are opened together, but after 4 hours, pipe C is closed. How much more time will it take to fill the tank completely?
    A 6 hours Correct Answer Incorrect Answer
    B 5.5 hours Correct Answer Incorrect Answer
    C 7.2 hours Correct Answer Incorrect Answer
    D 7 hours Correct Answer Incorrect Answer
    E 6.5 hours Correct Answer Incorrect Answer

    Solution

    Work done by A in 1 hour = 1/15 Work done by B in 1 hour = 1/20 Work done by C in 1 hour = 1/25 Work done by A, B, and C together in 1 hour = 1/15 + 1/20 + 1/25 = (20 + 15 + 12) / 300 = 47/300 Work done by A, B, and C in 4 hours = 4 × (47/300) = 188/300 = 47/75 Remaining work = 1 - 47/75 = 28/75 Now, only A and B are working. Work done by A and B in 1 hour = 1/15 + 1/20 = (4 + 3) / 60 = 7/60 Time taken to complete the remaining work = (28/75) ÷ (7/60) = (28/75) × (60/7)  = 3.2 hours Total time taken = 4 hours (initial) + 3.2 hours = 7.2 hours Correct Option: c

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