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      Question

      A man can go on a motorboat with a speed of 18 km/hour

      in still water to a certain upstream point and return to the starting point in a river that flows at a speed of 14 km/hour. Find the average speed of the motorboat for the total journey.
      A 27/4 Correct Answer Incorrect Answer
      B 64/9 Correct Answer Incorrect Answer
      C 22/9 Correct Answer Incorrect Answer
      D 52/9 Correct Answer Incorrect Answer

      Solution

      Let the speed of the boat be b km/hour and the speed of the current be s km/hour. Average speed = 2xy/ (x+ y) (here, x is the speed of downstream and y is the speed of upstream)   = (2 (b +s) (b-s))/ ((b+ s +b -s)) = (b2 – s2)/b = (324 - 196)/18   = 128/18   = 64/9 Km/h

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