A person can row 71/2 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:

Answer: Option B

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

$$\begin{array}{rl}& \text{According}\text{to}\text{question,}\\ & \frac{2x}{\frac{15}{2}+y}=\frac{x}{\frac{15}{2}-y}\\ & or,15-2y=\frac{15}{2}+y\\ & or,3y=15-\frac{15}{2}=\frac{15}{2}\\ & or,y=\frac{15}{6}=2.5\text{kmph}\end{array}$$