Discussion
A man's speed with the current is 15 km/hr and the speed of the current is 2.5 km/hr. The man's speed against the current is:
Answer: C
Write \( b \) for the boat's speed in still water and \( r \) for the stream's. Then
\[ \text{downstream} = b + r, \qquad \text{upstream} = b - r \]
"With the current" is the downstream speed, so
\[ b + r = 15, \qquad r = 2.5 \quad\Longrightarrow\quad b = 12.5 \text{ km/h} \]
\[ b - r = 12.5 - 2.5 = 10 \text{ km/h} \]
Equivalently, the current is subtracted twice in going from downstream to upstream: \( 15 - 2 \times 2.5 = 10 \). Subtracting it only once gives 12.5 - the still-water speed, offered as the last distractor.
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