Discussion

A man rows his boat 85 km downstream and 45 km upstream, taking 2 1/2 hours each time. Find the speed of the stream?

Answer: D

With \( b \) the still-water speed and \( r \) the stream's,

\[ \text{downstream} = b + r, \qquad \text{upstream} = b - r \]

\[ b + r = \frac{85}{2.5} = 34 \text{ kmph}, \qquad b - r = \frac{45}{2.5} = 18 \text{ kmph} \]

\[ r = \frac{34 - 18}{2} = 8 \text{ kmph} \]

The boat itself does 26 kmph in still water. Halving the difference is the step that matters - the full 16 kmph gap is twice the current.

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