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Problem 342: (2023 AMC 10a, 25; 12a 21)
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 $d(Q, R) > d(R, S)$? $\textbf{(A)}~\frac{7}{22}\qquad$ $\textbf{(B)}~\frac13\qquad$ $\textbf{(C)}~\frac38\qquad$ $\textbf{(D)}~\frac5{12}\qquad$ $\textbf{(E)}~\frac12$
Solution
Exercise 342: Solution
In a regular octahedron, define the distance $d(A,B)$ between two vertices to be the minimum number of edges one must traverse to get from $A$ to $B$. Let $Q$, $R$, and $S$ be randomly chosen distinct vertices of a regular octahedron. What is the probability that $d(Q,R)>d(R,S)$?
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