A man can row 15 km/h in still water and he finds that it takes him twice as much time to row up than as to row down the same distance in the river. The speed of current (in km/h) :

Explanation:-

Let the speed of the current be x km/h. Now, according to the question,
^(15+x)/_(15-x) = ²/₁.
15 + x = 30 - 2x
3x = 30 - 15
3x = 15
x = 5 km/h.