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      Question

      The speed of a boat while going downstream is 24 km/h, and

      while going upstream, it is 16 km/h. If the boatman takes 12 hours to cover (m + n) km upstream when the stream flows at twice its normal speed, find the average of m and n.
      A 72 km Correct Answer Incorrect Answer
      B 60 km Correct Answer Incorrect Answer
      C 93 km Correct Answer Incorrect Answer
      D 85 km Correct Answer Incorrect Answer
      E None of these Correct Answer Incorrect Answer

      Solution

      ATQ,

      Let speed of boat in still water and stream speed be B km/hour and s km/hour, respectively.

      So, B + s = 24 … (i)

      B - s = 16 … (ii)

      Solving equation (i) and (ii), we get,

      B = 20 km/hour

      s = 4 km/hour

      According to the question,

      (m + n)/(B - 2s) = 12

      (m + n) = 12 × (20 - 2 × 4) = 12 × 12 = 144 km

      Therefore, average value of m and n = (m + n)/2 = 144/2 = 72 km

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