A boat takes 2 hours to go 30 km downstream and 3 hours to come back. How long would it take to go 30 km in still water?

Explanation

1. 01

   Downstream 15 km/h, upstream 10 km/h, so still water $\frac{15 + 10}{2} = 12.5$ km/h.

2. 02

   $\frac{30}{12.5} = 2.4$ hours.