Discussion
A person can swim in still water at 4 km/h. If the speed of water 2 km/h, how many hours will the man take to swim back against the current for 6km?
Answer: A
Write \( b \) for the boat's speed in still water and \( r \) for the stream's. Then
\[ \text{downstream} = b + r, \qquad \text{upstream} = b - r \]
Swimming back against the current is the upstream leg:
\[ b - r = 4 - 2 = 2 \text{ km/h} \]
\[ t = \frac{6}{2} = 3 \text{ hours} \]
All the data needed is present, so "insufficient data" is a trap. Using the still-water speed of 4 km/h would give 1.5 hours - too optimistic, since the current is working against him.
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