13. (12 points) $A$ is 60 kilometers west of $B$. Jia, Yi start from $A$, and Bing, Ding start from $B$ at the same time. Jia, Yi, and Ding all travel east, while Bing travels west. It is known that the speeds of Jia, Yi, Bing, and Ding form an arithmetic sequence, with Jia being the fastest. After $n$ hours, Yi and Bing meet, and after another $n$ hours, Jia catches up with Ding at point $C$. The distance between $B$ and $C$ is ( ) kilometers.
A. 15
B. 30
C. 60
D. 90
【Solution】Solution: In $n$ hours, $S_{\text {乙 }}+S_{\text {丙 }}=60$ kilometers
In $2 n$ hours, $S_{\text {乙 }}+S_{\text {丙 }}=120$ kilometers
Let the distance difference between 甲, 乙, 丙, and 丁 in $2 n$ hours be $S_{0}$,
Then $S_{\text {乙 }}=S_{\text {甲 }}-S_{0}, S_{\text {丙 }}=S_{\mathrm{J}}+S_{0}$,
In $2 n$ hours, 甲 catches up with 丁: $S_{\text {甲 }}-S_{\mathrm{J}}=60$ kilometers
Adding (1) and (2) gives $S$ 甲 $=90$ kilometers, $S_{\mathrm{T}}=30$ kilometers
The distance between $B C$ is exactly $S_{\mathrm{T}}$,
Answer: The distance between $B C$ is 30 kilometers.