Question 130942
Solution-
   The format of these type (BOAT ,UPSTREAM AND DOWNSTREAM)is given below.you just have to substitute the values in the given format and solve it according to the condition. Here is the format
      let the speed of the boat  be x km/h
      and the speed of the river/stream be y km/h
      speed of boat in downstream = (x+y)km/h
      speed of boat in upstream = (x-y)km/h
      time taken by boat in downstream = distance for downstream/(x+y)
      time taken by boat in upstream = diatance for upstream/(x-y)km/h
       now solve according to the condition
 
 HERE IS THE SOLUTION OF YOUR QUESTION
     the speed of the boat is 12km/h
     let the speed of the river be y km/h
     speed of the boat in downstream = (12+y)km/h
     speed of the boat in upstream =(12-y)km/h
     time taken by boat in downstream = distance for downstream/(12+y)
                                      = 6/(12+y)
     time taken by boat in upstream = distance for upstream/(12-y)
                                    = 3/(12-y)
     now according to the condition 
   time taken by boat in downstream = time taken by boat in upstream
        6/(12+y)= 3/(12-y)
      now by cross multiplication we have
       6(12-y)=3(12+y)
       72-6y = 36+3y
     shifting the like terms on one side we have
       -6y-3y = 36-72
         -9y = -36
           y = 4
 Therefore the speed of the river is 4 km/h

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