A boat goes 40 km downstream in 2 hours. The stream flows at 4 km/h. How long does the return journey take?

Explanation

1. 01

   Downstream speed $\frac{40}{2} = 20$ km/h, so the boat's own speed is $20 - 4 = 16$ km/h and upstream it goes $16 - 4 = 12$ km/h.

2. 02

   $\frac{40}{12} = \frac{10}{3}$ hours, that is 3 hours 20 minutes.