Discussion
A boat running downstream covers a distance of 20 km in 2 hours while for covering the same distance upstream, it takes 5 hours. What is the speed of the boat in still water?
Answer: B
Write \( b \) for the boat's speed in still water and \( r \) for the stream's. Then
\[ \text{downstream} = b + r, \qquad \text{upstream} = b - r \]
\[ b + r = \frac{20}{2} = 10 \text{ km/h}, \qquad b - r = \frac{20}{5} = 4 \text{ km/h} \]
\[ b = \frac{10 + 4}{2} = 7 \text{ km/h} \]
The data are entirely sufficient, so "data inadequate" is a trap: two journey times over a known distance always determine both speeds.
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