A boat takes 3 hours to go 36 km downstream and 6 hours to come back. How long would it take to go 36 km in still water?

Explanation

1. 01

   Downstream $\frac{36}{3} = 12$ km/h, upstream $\frac{36}{6} = 6$ km/h, so still water $\frac{12 + 6}{2} = 9$ km/h.

2. 02

   $\frac{36}{9} = 4$ hours.