Question 1084421
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The effective speed upstream is {{{10/2}}} = 5 km/h.
It is the <U>difference</U> of the boat speed in still water "u" and the rate of stream "v":

u - v =  5.    (1)


The effective speed downstream is {{{30/2}}} = 15 km/h.
It is the <U>sum</u> 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 = {{{20/2}}} = 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).


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