A man swims 12 km downstream and 10 km upstream. If he takes 2 hours each time, what is the speed of the stream?

0.5 Km/hr

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,

$s = 6 - 5.5 = 0.5\text{ km/h}$