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      Question

      Speed of boat in downstream is 45% more than speed of

      boat in still water. If speed of stream is 9 km/h, then find the time taken by the boat to cover 81 km in downstream and 54 km in upstream.
      A 10 hrs Correct Answer Incorrect Answer
      B 7.7 hrs Correct Answer Incorrect Answer
      C 6.5 hrs Correct Answer Incorrect Answer
      D 8.9 hrs Correct Answer Incorrect Answer
      E None of these Correct Answer Incorrect Answer

      Solution

      Let speed of boat in still water be тАШcтАЩ km/h. Therefore, downstream speed of the boat = 1.45c km/h. Speed of stream = 1.45c - c = 0.45c km/h. So, 0.45c = 9. Or, c = 20. So, speed of boat in still water = 20 km/h. Upstream speed of boat = 20 - 9 = 11 km/h. Downstream speed = 20 + 9 = 29 km/h. Desired time = (81/29) + (54/11) = 2.79 + 4.91 = 7.7 hours.

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