AMC 10 Step by Step
2023 AMC 10AProblem 25P21-25~8 minPrint

If AA and BB are vertices of a polyhedron, define the distance d(A,B)d(A, B) to be the minimum number of edges of the polyhedron one must traverse in order to connect AA and BB . For example, AB\overline{AB} is an edge of the polyhedron, then d(A,B)=1d(A, B) = 1 , but if AC\overline{AC} and CB\overline{CB} are edges and AB\overline{AB} is not an edge, then d(A,B)=2d(A, B) = 2 . Let QQ , RR , and SS be randomly chosen distinct vertices of a regular icosahedron (regular polyhedron made up of 2020 equilateral triangles). What is the probability that d(Q,R)>d(R,S)d(Q, R) > d(R, S) ?

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Problem © Mathematical Association of America (MAA), American Mathematics Competitions. Reproduced for non-commercial educational use. Solution and commentary are original to this site.