79. A small boat sails upstream from downstream port $A$, and at a point 4 kilometers away from port $A$, it encounters a log drifting with the current. The boat continues its journey to reach upstream port $B$ and immediately turns around, arriving at port $A$ at the same time as the log. Given that the speed of the current is 2 meters/second, and the speed of the boat in still water is 8 meters/second, the distance between ports $A$ and $B$ is $\qquad$ kilometers.
Reference answer: 8
Key points: Travel problems - meeting and pursuit, variable speed problems