A person can row $$7\frac{1}{2}$$ km an hour in still water. Finds that it takes twice the time to row upstream than the time to row downstream. The speed of the stream is:

Let the distance covered be x km and speed of stream = y kmph.
Speed downstream = $$\frac{{15}}{2} + y$$   kmph
Speed upstream = $$\frac{{15}}{2} - y$$   kmph

$$\eqalign{ & {\text{According}}\,{\text{to}}\,{\text{question,}} \cr & {\frac{{2x}}{{ { {\frac{{15}}{2}} + y} }}} = {\frac{x}{{ { {\frac{{15}}{2}} - y} }}} \cr & or,\,15 - 2y = {\frac{{15}}{2}} + y \cr & or,3y = 15 - {\frac{{15}}{2}} = \frac{{15}}{2} \cr & or,y = \frac{{15}}{6} = 2.5\,{\text{kmph}} \cr} $$