If $A$ and $B$ are vertices of a polyhedron, define the distance $d(A,B)$ to be the minimum number of edges of the polyhedron one must traverse in order to connect $A$ and $B$. For example, if $\overline{AB}$ is an edge of the polyhedron, then $d(A, B) = 1$, but if $\overline{AC}$ and $\overline{CB}$ are edges and $\overline{AB}$ is not an edge, then $d(A, B) = 2$. Let $Q$, $R$, and $S$ be randomly chosen distinct vertices of a regular icosahedron (regular polyhedron made up of 20 equilateral triangles). What is the probability that $d(Q, R) > d(R, S)$?
若 $A$ 和 $B$ 是多面体的顶点,定义距离 $d(A,B)$ 为连接 $A$ 和 $B$ 所需穿越的最少边数。例如,若 $\overline{AB}$ 是多面体的一条边,则 $d(A, B) = 1$;若 $\overline{AC}$ 和 $\overline{CB}$ 是边而 $\overline{AB}$ 不是,则 $d(A, B) = 2$。设正二十面体(由 $20$ 个正三角形组成的正多面体)的三个不同顶点 $Q$、$R$ 和 $S$ 随机选取。求 $d(Q, R) > d(R, S)$ 的概率。