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:
a) 2 kmph
b) 2.5 kmph
c) 3 kmph
d) 4 kmph
Answer: b
Explanation:
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
$$\eqalign{
& {\text{According}}\,{\text{to}}\,{\text{question,}} \cr
& {\frac{{2x}}{{ { {\frac{{15}}{2}} + y} }}} = {\frac{x}{{ { {\frac{{15}}{2}} – y} }}} \cr
& or,\,15 – 2y = {\frac{{15}}{2}} + y \cr
& 3y = 15 – {\frac{{15}}{2}} = \frac{{15}}{2} \cr
& y = \frac{{15}}{6} = 2.5\,{\text{kmph}} \cr} $$
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