The effective speed upstream is  = 5 km/h.
It is the difference of the boat speed in still water "u" and the rate of stream "v":

u - v =  5.    (1)


The effective speed downstream is  = 15 km/h.
It is the sum of the boat speed in still water "u" and the rate of stream "v":

u + v = 15.    (2)


Add the equations (1) and (2). You will get

2u = 5 + 15 = 20,   --->  u =  = 10 km/h.


Thus the speed of the boat in still water is 10 km/h.

Then the speed of the stream is v = 15 - 5 = 15 - 10 = 5 km/h, from equation (2).


Answer.  The speed of the boat in still water is 10 km/h.  The speed of the stream is 5 km/h.