If and are vertices of a polyhedron, define the distance to be the minimum number of edges of the polyhedron one must traverse in order to connect and . For example, is an edge of the polyhedron, then , but if and are edges and is not an edge, then . Let , , and be randomly chosen distinct vertices of a regular icosahedron (regular polyhedron made up of equilateral triangles). What is the probability that ?
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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.