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      Question

      Pipe A and Pipe B could fill a tank together in 40 hours

      and 16 hours respectively, while pipe C could empty it in 20 hours. If both inlet pipes are opened together for first 4 hours, then pipe C replaced them. Find the time taken by pipe C to empty the tank filled by pipe A and pipe B together.
      A 6 hr 45 min Correct Answer Incorrect Answer
      B 7 hr Correct Answer Incorrect Answer
      C 6 hr 30 min Correct Answer Incorrect Answer
      D 5 hr 40 min Correct Answer Incorrect Answer
      E 7 hr 15 min Correct Answer Incorrect Answer

      Solution

      Let the capacity of the tank be 80 units. (LCM of 40, 16 and 20)
      Efficiency of pipe A = 80/40 = 2 units/hr
      Efficiency of pipe B = 80/16 = 5 units/hr
      Efficiency of pipe C = 80/20 = –4 units/hr ATQ,
      Tank filled by pipe A and B together in 4 hours = (2 + 5) Ă— 4 = 28 units
      Required time = 28/4 = 7 hr

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