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?