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      Question

      Pipes D and E alone can fill a tank in 5 hours and 3 hours

      respectively. When both pipes along with pipe F, an outlet pipe, are opened together, 1/2 of the tank gets filled in 60 minutes. Find the time taken by pipe F to empty a completely filled tank.
      A 20 hours Correct Answer Incorrect Answer
      B 25 hours Correct Answer Incorrect Answer
      C 30 hours Correct Answer Incorrect Answer
      D 35 hours Correct Answer Incorrect Answer
      E 40 hours Correct Answer Incorrect Answer

      Solution

      Let the total capacity of the tank be 15 litres.
      Efficiency of Pipe 'D' = 15 / 5 = 3 litres/hour
      Efficiency of Pipe 'E' = 15 / 3 = 5 litres/hour
      Let pipe 'F' empties the tank with an efficiency of 'm' litres/hour.
      Tank filled in 60 minutes = (1/2) × 15 = 7.5 litres
      ATQ,
      (3 + 5 - m) × (60/60) = 7.5
      (8 - m) × 1 = 7.5
      8 - m = 7.5
      m = 0.5 litres/hour
      Time taken by F to empty the tank = 15 / 0.5 = 30 hours

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