7. As shown in the figure, an ant is crawling on a grid, and each step it takes means moving from one point to an adjacent point (two points connected by a dashed line segment are called adjacent points). The ant is going to crawl four steps in total. If it starts from point $A$, there are $m$ different crawling routes; if it starts from point $B$, there are $n$ different crawling routes. Then $n \div m=$ $\qquad$ .
【Answer】 3
【Solution】 We find that, whether starting from point $A$ or point $B$, the next step is to walk to a position like point $C$ (the six red points in the figure below). According to symmetry, the number of ways to walk from each red point is the same. Point $A$ has two ways to walk to a red point, and point $B$ has six ways to walk to a red point, so $n \div m=6 \div 2=3$.

【Explanation】 Symmetric counting