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      Question

      There are two inlet pipes 'P', 'Q' and one outlet pipe 'R'

      in a tank. ‘P’ and ‘Q’ can fill the tank in 20 minutes and 10 minutes respectively. ‘R’ is 50% less efficient than ‘P’. If ‘P’ and ‘R’ are open together but after 10 minutes, ‘P’ is replaced with ‘Q’, find the total time required to fill the tank completely.
      A 15 minutes Correct Answer Incorrect Answer
      B 18 minutes Correct Answer Incorrect Answer
      C 20 minutes Correct Answer Incorrect Answer
      D 22 minutes Correct Answer Incorrect Answer
      E 25 minutes Correct Answer Incorrect Answer

      Solution

      Tank capacity = 20x units (LCM of 20 and 10).
      P = x units/min, Q = 2x units/min, R = 0.5x units/min
      First 10 mins: (x - 0.5x) × 10 = 5x units
      Remaining = 20x - 5x = 15x units
      Net rate = 2x - 0.5x = 1.5x
      Time = 15x / 1.5x = 10 minutes
      Total time = 10 + 10 = 20 minutes

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