Suppose you are rowing a boat upstream as fast as you can. The water is flowing at 2 miles per hour, and your speed against the current is 3 miles per hour. Your hat blows off your head and into the water, but you don’t notice. Fifteen minutes later, feeling the heat of the sun on your head, you realize your hat is gone. You turn around immediately and row back downstream as fast as you can. How long does it take for you to reach your hat?

It's one quarter of an hour. :-)
To start with your hat floats away at 2mph and you row away at 3mph. That is a net distancing from your hat at 5mph for 15mins. That's a mile and a quarter. In order to make 3mph against a 2mph current you have to be able to row at 5mph. So that is your speed after you turn have turned around. Both you and the hat also move down the river at 2mph but you can ignore that. Your relative speed with respect to the hat (your catch up speed) is 5mph and you are separated from your hat by one and a quarter miles. That will take 15mins to do at 5mph.
The question is unclear - you might be rowing against the river at 3mph and make a net 1mph against it. It doesn't matter, the answer will still be 15mins.