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      Question

      Pipe A and Pipe B could fill a tank together in 30 hours

      and 20 hours respectively, while pipe C could empty it in 12 hours. If both inlet pipes are opened together for first 6 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 5 hr 30 min Correct Answer Incorrect Answer
      B 6 hr 30 min Correct Answer Incorrect Answer
      C 4 hr 45 min Correct Answer Incorrect Answer
      D 6 hr Correct Answer Incorrect Answer

      Solution

      Let the capacity of the tank be 60 units. (LCM of 30, 20 and 12)
      Efficiency of pipe A = 60/30 = 2 units/hr
      Efficiency of pipe B = 60/20 = 3 units/hr
      Efficiency of pipe C = 60/12 = –5 units/hr (β€˜βˆ’β€™ sign, since it’s an outlet pipe) ATQ,
      Tank filled by pipe A and B together in 6 hours = (2 + 3) Γ— 6 = 30 units
      Required time = 30/5 = 6 hr

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