12. (6 points) A passenger liner has a navigation speed of 26 km/h in still water, traveling back and forth between ports $A$ and $B$. The river current speed is 6 km/h. If the liner makes 4 round trips in a total of 13 hours, then the distance between ports $A$ and $B$ is $\qquad$ kilometers. (Neglect the time for the liner to turn around)


【Solution】Solution: The round trip distance is constant, the speed ratio is $(26+6):(26-6)=8:5$, so the time ratio for the trips is: 5:8. The time used for one return trip is: $13 \div 4=3.25$ (hours), so the time used for the downstream trip is: $3.25 \times \frac{5}{5+8}=1.25$ (hours). Therefore, the distance between the two ports is: $1.25 \times (26+6) = 40$ (km); Answer: The distance between the two ports is 40 km. Therefore, the answer is: 40 .