A ferry moves at 9 km/h in still water and the river flows at 3 km/h. How long does it take to go 24 km upstream and come back?

Explanation

1. 01

   Upstream speed = 6 km/h, downstream speed = 12 km/h.

2. 02

   Times: $\frac{24}{6} = 4$ h and $\frac{24}{12} = 2$ h.

3. 03

   Total = 6 h.