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      Question

      Pipe A can fill a tank in 15 minutes and Pipe B can fill

      it in 20 minutes. Both pipes are opened together for 4 minutes. After that, Pipe A is closed and Pipe C, which can empty the full tank in 30 minutes, is opened along with Pipe B. In how many more minutes will the tank be filled?
      A 25 mins Correct Answer Incorrect Answer
      B 32 mins Correct Answer Incorrect Answer
      C 48 mins Correct Answer Incorrect Answer
      D 21 mins Correct Answer Incorrect Answer
      E None of these Correct Answer Incorrect Answer

      Solution

      A's 1-minute work = 1/15 B's 1-minute work = 1/20 Together, A + B = 1/15 + 1/20 = 4/60 + 3/60 = 7/60 Work done in 4 minutes = 4 x 7/60 = 28/60 = 7/15 Remaining work = 1 - 7/15 = 8/15 Now Pipe A is closed and Pipe C is opened. B's 1-minute work = 1/20 C's 1-minute work = -1/30 Net work by B and C = 1/20 - 1/30 = 3/60 - 2/60 = 1/60 Time to complete remaining work = (8/15) ÷ (1/60) = (8/15) x 60 = 32 minutes Hence, the correct answer is 32 minutes.

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