A boat covers 30 km downstream in 2 hours and the same 30 km upstream in 3 hours. What is its speed in still water?

Explanation

1. 01

   Downstream speed $= \frac{30}{2} = 15$ km/h; upstream $= \frac{30}{3} = 10$ km/h.

2. 02

   Still water $= \frac{15 + 10}{2} = 12.5$ km/h.