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      Question

      Pipe β€˜A’ and pipe β€˜B’ can fill a cistern in 18

      minutes and 20 minutes respectively. Pipe β€˜C’ alone can empty the cistern in 12 minutes. If all three pipes are opened together then what is the time taken to fill 50% of the cistern?
      A 15 minutes Correct Answer Incorrect Answer
      B 5 minutes Correct Answer Incorrect Answer
      C 22.5 minutes Correct Answer Incorrect Answer
      D 7.5 minutes Correct Answer Incorrect Answer

      Solution

      Let the capacity of the cistern = 180 units Then, efficiency of pipe β€˜A’ = 180/18 = 10 units/minute Efficiency of pipe β€˜B’ = 180/20 = 9 units/minute Efficiency of pipe β€˜C’ = 180/12 = 15 units/minute So, combined efficiency of pipes β€˜A’, β€˜B’ and β€˜C’ = 10 + 9 – 15 = 4 units/minute 50% of the cistern’s capacity = 90 units Therefore, time taken by all 3 pipes together to fill 50% of the cistern = 90/4 = 22.5 minutes

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