Rowing upstream, Tenzin drops his cap into the river. He notices 20 minutes later, turns round at once and rows downstream at the same effort. He picks up the cap 2 km downstream of where it fell. What is the speed of the river?

Explanation

1. 01

   Relative to the water, the cap is still and Tenzin rows at the same speed away and back.

2. 02

   So he takes 20 minutes to get back to the cap: 40 minutes in all after the drop.

3. 03

   In those 40 minutes the cap drifts 2 km with the current.

4. 04

   River speed $= 2 \div \frac{40}{60} = 3$ km/h.