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      Question

      A boat moves in a river where the ratio of the speed of

      the stream to the downstream speed of the boat is 1:6. If it takes the boat a total of 20 hours to travel 144 km downstream and return 144 km upstream, how much time will it take to cover 180 km in still water?
      A 22 hrs Correct Answer Incorrect Answer
      B 10 hrs Correct Answer Incorrect Answer
      C 12 hrs Correct Answer Incorrect Answer
      D 24 hrs Correct Answer Incorrect Answer
      E None of these Correct Answer Incorrect Answer

      Solution

      ATQ,

      Let the downstream speed of a boat and the speed of stream be 6x km/hr and 'x' km/hr respectively.

      Or, upstream speed of boat = 6x - x - x = 4x km/hr

      ATQ,

      (144/6x) + (144/4x) = 20

      Or, (24/x) + (36/x) = 20

      Or, (60/x) = 20

      So, 'x' = 3

      So, speed of boat in still water = 6x - x = 5x = 5 × 3 = 15 km/hr

      So, required time = (180/15) = 12 hours

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