A mathematics teacher was getting his exercise by rowing a b

A mathematics teacher was getting his exercise by rowing a boat upstream. As he passed under a low bridge, his hat was knocked off. It was not, however, until 5 minutes later that he noticed his hat was missing. At this time he turned around and rowed with the same effort downstream. He retrieved his hat at a point precisely one mile downstream from the offending bridge. How fast was the current flowing?

Solution

His relationship to the hat is not affected by the current.
It would take him 5 min to return to the point where the hat is in the water.
That point in relationship to the bridge will have traveled one mile in 10 minutes.
( 5 min to discover the hat missing, and 5 min to return to the hat)
:
C = 1 mi in 10 min
then
C = 6 mph is the current