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      Question

      An inlet pipe can fill a cistern in 25 minutes when

      working alone. However, due to a leak, the fully filled cistern would empty in 87.5 minutes. If the leak is not fixed, how long will it take for the inlet pipe to fill 60% of the empty cistern?
      A 15 minutes Correct Answer Incorrect Answer
      B 21 minutes Correct Answer Incorrect Answer
      C 30 minutes Correct Answer Incorrect Answer
      D 24 minutes Correct Answer Incorrect Answer

      Solution

      Let the capacity of the cistern = L.C.M of 25 and 87.5 = 175 units

      So, efficiency of the inlet pipe = 175 Γ· 25 = 7 units/minute

      Efficiency of the leak = 175 Γ· 87.5 = 2 units/minute (outlet)

      So, combined efficiency of the inlet pipe with the leakage = 7 - 2 = 5 units/minute

      60% of the cistern = 175 X 0.6 = 105 units

      Time taken for the inlet pipe to fill 60% of the cistern with the leakage = 105 Γ· 5 = 21 minutes

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