Discussion

In a river, a man takes 3 hours in rowing 3 km upstream or 15 km downstream. What is the speed of the current?

Answer: A

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

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

Both legs take 3 hours, so read each speed straight off its distance.

\[ b - r = \frac{3}{3} = 1 \text{ km/hr}, \qquad b + r = \frac{15}{3} = 5 \text{ km/hr} \]

\[ r = \frac{5 - 1}{2} = 2 \text{ km/hr} \]

The boat does 3 km/hr in still water - only slightly faster than the current, which is why the upstream progress is so slow.

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