13. (15 points) A speedboat departs from dock $A$, travels downstream along the river, passes through dock $B$, and continues downstream to dock $C$. Upon reaching dock $C$, it immediately turns around and heads back to dock $B$, taking a total of 10 hours. If the distance between $A$ and $B$ is 20 kilometers, the speed of the speedboat in still water is 40 kilometers/hour, and the speed of the river current is 10 kilometers/hour, find the distance between $B$ and $C$.
【Solution】Solution: Let the distance between $B$ and $C$ be $x$ kilometers. According to the problem, we have
$$
\frac{20+x}{40+10}+\frac{x}{40-10}=10 \text{, }
$$

Solving for $x$ gives $x=180$.
Answer: The distance between $B$ and $C$ is 180 kilometers.