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      Question

      Pipes A and B can fill a cistern in 48 and 80 minutes

      respectively. Another pipe C can empty the cistern in 1 hour 36 minutes. A and C are opened first. After 30 minutes, B is turned on. Find the total time taken to fill the cistern.
      A 75 mins Correct Answer Incorrect Answer
      B 27 mins Correct Answer Incorrect Answer
      C 60 mins Correct Answer Incorrect Answer
      D 65 mins Correct Answer Incorrect Answer
      E None of these Correct Answer Incorrect Answer

      Solution

      ATQ,

      Let capacity = 480 litres (LCM of 48, 80, 96)

      Efficiency of A = 480 Γ· 48 = 10 litres/minute

      Efficiency of B = 480 Γ· 80 = 6 litres/minute

      Efficiency of C = 480 Γ· 96 = -5 litres/minute

      Filled in 30 minutes = (10 - 5) Γ— 30 = 150 litres

      Remaining = 480 - 150 = 330 litres

      Time = 330 Γ· (10 + 6 - 5) = 330 Γ· 11 = 30 minutes

      Total time = 30 + 30 = 60 minutes

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