Discussion
A man swims downstream 30 km and upstream 18 km taking 3 hours each time, what is the speed of the man in still water?
Answer: B
With \( b \) the still-water speed and \( r \) the stream's,
\[ \text{downstream} = b + r, \qquad \text{upstream} = b - r \]
\[ b + r = \frac{30}{3} = 10 \text{ kmph}, \qquad b - r = \frac{18}{3} = 6 \text{ kmph} \]
\[ b = \frac{10 + 6}{2} = 8 \text{ kmph} \]
Because both legs take the same 3 hours, the distances are directly proportional to the speeds - which is what makes the two speeds so easy to read off.
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