There is a road between A and B. A starts from A and walks to B, and B rides a motorcycle from B to A and B continuously. If they start at the same time, the two meet for the first time after 80 minutes, and B overtakes A for the first time after 100 minutes. Ask how many times B has caught up with A when A arrives at B.
A、B两地间有一条公路，甲从A地出发步行到B地，乙骑摩托车从B地不停地往返A、B两地之间．如他们同时出发，80分钟后两人第一次相遇，100分钟后乙第一次超过甲，问当甲到达B地时，乙追上甲多少次．
<result>4</result>