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      Question

      A motor boat goes downstream from point A to B ,which

      is 36 km away from point A, and then returns to A.If actual speed of the boat in still water is 7km/hour ,and the total upstream and downstream journey takes 21 hours .What must be the actual speed of boat for the journey from A to B to take exactly 108 minutes ?
      A 18 km/hr Correct Answer Incorrect Answer
      B 12 km/hr Correct Answer Incorrect Answer
      C 15 km/hr Correct Answer Incorrect Answer
      D 21 km/hr Correct Answer Incorrect Answer
      E 30 km/hr Correct Answer Incorrect Answer

      Solution

      Speed of boat in still water is 7km/hour  Let the Speed of Stream be x Km/hour  36/(7+x) + 36/(7-x) = 21 24 = 49- x^2 x = 5 km/hour  Now, the speed of Boat for the journey from A to B to take time 108 minutes is   Speed of boat = y km/hour   ,  Speed of stream =5km/hour  ,  Time = 108 minutes (y+5) =  (36×60)/108            = 20km/hour   y+5 = 20     y = 15km/hour ∴ Speed of Boat must be 15km/hour

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