13. (12 points) Cars A and B start from point A to point B at the same time. At the start, car A's speed is 2.5 kilometers per hour faster than car B's. After 10 minutes, car A slows down; 5 minutes later, car B also slows down, at which point car B is 0.5 kilometers per hour slower than car A. Another 25 minutes later, both cars arrive at point B at the same time. By how many kilometers per hour did car A's speed decrease at that time?
13. (12 points) Two cars, A and B, start from point A to point B at the same time. At the start, car A is 2.5 km/h faster than car B. After 10 minutes, car A slows down; 5 minutes later, car B also slows down, at which point car B is 0.5 km/h slower than car A. Another 25 minutes later, both cars arrive at point B at the same time. Car A's speed was reduced by 10 km/h at that time.
【Solution】: 10 minutes $=\frac{1}{6}$ hour
25 minutes $=\frac{5}{12}$ hour
5 minutes $=\frac{1}{12}$ hour
In the first 10 minutes, car A travels $\frac{5}{12}$ (km) more than car B: $2.5 \times \frac{1}{6}=\frac{5}{12}$ (km)
In the last 25 minutes, car A travels $\frac{5}{24}$ (km) more than car B: $0.5 \times \frac{5}{12}=\frac{5}{24}$ (km)
In the middle 5 minutes, car B should travel $\frac{5}{8}$ (km) more than car A: $\frac{5}{12}+\frac{5}{24}=\frac{5}{8}$ (km)
In the middle 5 minutes, car B is faster than car A by: $\frac{5}{8} \div \frac{1}{12}=7.5$ (km/h)
Car A's speed reduction: $2.5+7.5=10$ (km/h)
Answer: Car A's speed was reduced by 10 km/h at that time.
Therefore, the answer is: 10.