Question 805445
A man is rowing a boat upstream. As he passes under a bridge, a bottle falls out. 20 min later the rower realizes this and rows back to pick it up. He catches up with the bottle 1 mile downstream from the bridge. How fast is the current?
======================
Choose the frame of reference to simplify things.
Ignore the bridge.
========================
He rows away from the bottle for 20 minutes, so it takes him 20 minutes to row back to it, 40 minutes total.
---
In 40 minutes (2/3 hours), the bottle moved 1 mile.
1/(2/3) = 1.5 mi/hr current
---------------------------
His direction is also irrelevant, he can row up or downstream, or any direction.
The situation is similar if the bottle falls out near a buoy in open water.