A boat's downstream speed and upstream speed are in the ratio $5 : 3$. If its speed in still water is 8 km/h, what is the speed of the stream?

Explanation

1. 01

   Let downstream = 5k and upstream = 3k.

2. 02

   Still water $= \frac{5k + 3k}{2} = 4k = 8$, so $k = 2$.

3. 03

   Stream $= \frac{5k - 3k}{2} = k = 2$ km/h.