Discussion
A man can row his boat with the stream at 6 km/h and against the stream at 4 km/h. The man's rate in still water is?
Answer: B
Write \( b \) for the boat's speed in still water and \( r \) for the stream's. Then
\[ \text{downstream} = b + r, \qquad \text{upstream} = b - r \]
\[ b + r = 6 \text{ km/h}, \qquad b - r = 4 \text{ km/h} \]
\[ b = \frac{6 + 4}{2} = 5 \text{ kmph} \]
The stream's own rate is \( \tfrac{6-4}{2} = 1 \) kmph, which is offered as a distractor. The man's rate must lie between his two observed speeds, so it has to be 5.
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