13. (15 points) Zhang Qiang rides a bike from bus station $A$, along the bus route, traveling 250 meters per minute. After a period of time, a bus also departs from station $A$, traveling 450 meters per minute, and needs to stop at a station for 1 minute after every 6 minutes of travel. If the bus catches up with Zhang Qiang 15 minutes after it departs, how far had Zhang Qiang already ridden when the bus departed?
【Solution】Solution: According to the problem, we know:
The bus travels 950 meters more than Zhang Qiang every 7 minutes, which is $(450-250) \times 6-250 \times 1=950$ (meters);
Since $15 \div 7=2 \cdots 1$ (min).
The bus can catch up with Zhang Qiang after traveling twice and then for another 1 minute. Therefore, the distance Zhang Qiang has traveled when the bus starts is:
$$
950 \times 2+(450-250)=2100 \text { (meters); }
$$

Another solution: In 15 minutes, Zhang Qiang has cycled: $250 \times 15=3750$ (meters);
The bus actually travels for $15-2=13$ (minutes), covering a distance of $450 \times 13=5850$ (meters).
In this time, the bus is 2100 meters behind Zhang Qiang: $5850-3750=2100$ (meters);
Answer: When the bus starts, the distance Zhang Qiang has already cycled is 2100 meters.