Question 766123
Let speed of water in river be u kmph
Let speed of boat in still water (water not moving) be v kmph

then time taken by boat to row DOWN the river will be

t*(v + u) = d  ...(1)

t = time taken to row down, speed of boat will be  (u+v) as flowing water will help boat go fast, d=distance traveled down stream 

Similarly time time taken by boat to row UP the river will be

4t*(v - u) = d   .....(2)

t = time taken to row UP which is 4 times the time taken to row DOWN, speed of boat will be  (v - u) as flowing water will slowdown the boat, d=distance traveled down stream 

As distance is same so equating both equations

t*(v + u) = 4t*(v - u)

This gives 5*u = 3*v

Now it is given that water speed is 6 kmph which means u = 6 kmph

substituting the value we get 

v = 10 kmph

Speed of boat in still water is 10 kmph