A launch travels 45 km downstream in 3 hours on a river flowing at 3 km/h. How long will it take to return the same 45 km upstream?

Explanation

1. 01

   Downstream speed $= \frac{45}{3} = 15$ km/h, so still-water speed = $15 - 3 = 12$ km/h.

2. 02

   Upstream speed = $12 - 3 = 9$ km/h.

3. 03

   Time $= \frac{45}{9} = 5$ h.