A boat's speed in still water is 20 km/h. Going 60 km upstream takes 1 hour 36 minutes longer than going 60 km downstream. What is the speed of the stream?

Explanation

1. 01

   1 h 36 min = 1.6 h. We need $\frac{60}{20 - s} - \frac{60}{20 + s} = 1.6$.

2. 02

   Try $s = 5$: $\frac{60}{15} - \frac{60}{25} = 4 - 2.4 = 1.6$. It fits.

3. 03

   A slower stream gives a smaller gap (for $s = 4$: $3.75 - 2.5 = 1.25$ h), so 5 km/h is the only answer.