man in rowboat
 

A man is rowing his boat, upstream, on a river spanned by two bridges one mile apart. As he passes under the more upstream bridge, a wind gust blows his hat into the water. He continues to row upstream for ten more minutes, then decides to turn around and retrieve his hat. He catches up to his hat just as he passes under the downstream bridge.

If he always rows at the same rate, and takes no time to turn around, how fast is the current in the river?

Man in rowboat
 

3 miles per hour. In ten minutes the hat will be half-way to the downstream bridge.