The boat in a river can go upstream at 25 km/h and downstream at 36 km/h. As the power of the motor is the same in both the cases, the difference in stream is due to the speed at which the river flows getting adding to that of the boat in the latter case and the same getting subtracted from the speed of the boat in the former case. Let the speed at which the river flows be X and the speed of the boat in still water be Y.
Y + X = 36 and Y - X = 25
Subtracting the second equation from the first,
Y + X - (Y - X) = 36 - 25
=> 2X = 11
=> X = 5.5
The speed at which the river flows is 5.5 km/h