Mathematics · Speed, Time and Distance

A girl was traveling in a boat when suddenly her hat fell into the water and started drifting backward with the stream. The boat kept moving for 12 more minutes against the stream before the girl realized her hat had fallen. She then turned around and rowed with the stream to catch her hat. If she caught the hat exactly at her starting point, and the hat had fallen into the water 3 km away from the starting point, what is the speed of the stream?