<PRE><B>Question 147415</B>
let the speed of boat in still water is x km/hr.

given speed of stream is  3 km/hr.

then speed of boat upstream = ( x - 3) km/hr
    speed  of boat downstream = (x + 3 ) km/hr

  Distance travelled upstream = 4 km
  time taken to travel upstream = distance travelled upstream / speed of boat                    
                                                                  upstream
                                = 4 / (x - 3 ) .............(1)


  Distance travelled downstream = 10 km
time taken to travel downstream = distance travelled downstream / speed of boat 
                                                                     downstream
                                  =  10 / (x + 3)   ............(2)
given that boat takes same time to travel upstream and downstream.i.e; (1) &amp; (2) are equal
         i.e. 4/(x-3)  = 10/ (x + 3 )

       4 * (x +3 ) = 10 * (x- 3)
        4x + 12 = 10x - 30
       12 + 30 = 10x -4x 
       42 = 6x 
      x = 42 / 6 = 7.

i.e speed of boat in still water is 7 km/hr.</PRE>