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      Question

      Pipe ‘A’ and pipe ‘B’,

      together can fill 20% of a tank in 7 hours while pipe ‘C’ takes 42 hours to empty it. Pipe ‘A’ and pipe ‘B’ were opened together. After 30 hours, pipe ‘C’ is also opened. Find the total time taken to fill the empty tank this way.
      A 60 hours Correct Answer Incorrect Answer
      B 42 hours Correct Answer Incorrect Answer
      C 50 hours Correct Answer Incorrect Answer
      D 45 hours Correct Answer Incorrect Answer
      E 35 hours Correct Answer Incorrect Answer

      Solution

      Time taken by pipe ‘A’ and pipe ‘B’, together to fill the tank = 7/0.20 = 35 hours
      Let the total capacity of tank = 210 units
      Efficiency of pipe (A + B) = 210/35 = 6 units/hour
      Efficiency of pipe ‘C’ = 210/42 = 5 units/hour
      Tank filled by pipe (A + B) in 30 hours = 30 × 6 = 180 units
      Time taken by all three pipes to fill the remaining tank = (210 – 180)/(6 – 5) = 30 hrs
      Total time taken = 30 + 30 = 60 hours

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