A man swims 12 km downstream and 10 km upstream. If he takes 2 hours each time, what is the speed of the stream?
Answer & explanation
Correct answer: option 4
Let:
- Speed of the swimmer in still water = $b$ km/h
- Speed of the stream = $s$ km/h
Given:
- Downstream: 12 km in 2 hours
$b + s = \frac{12}{2} = 6$
- Upstream: 10 km in 2 hours
$b - s = \frac{10}{2} = 5$
Adding the two equations:
$2b = 11 ⇒b = 5.5$
Therefore,
$s = 6 - 5.5 = 0.5\text{ km/h}$