After going for exactly one mile, his hat flips off of his head, and starts floating downstream. Bob doesn't realize that his hat has fallen off for five minutes, but then he notices that it's missing, and turns full throttle downstream.
He finally catches the hat at exactly the same spot as he camped that morning. The question is, how fast was the water traveling?
(Assume that he travels the same speed the entire time and that there is no turn around time.)