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2023 AMC 12A Problem 21

Problem 21 of 25HarderCombinatoricsProbability & StatisticsProblem-Solving Techniques

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 B.B. For example, if AB‾\overline{AB} is an edge of the polyhedron, then d(A,B)=1,d(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)=2.d(A,B)=2. Let Q,Q, R,R, 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)\gt d(R,S)?

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Solution

Fix R.R. Among the other 1111 vertices of the icosahedron, 55 are at distance 1,1, 55 are at distance 2,2, and 11 (the antipode) is at distance 3.3. Choosing ordered distinct Q,S,Q,S, the probability that d(Q,R)=d(R,S)d(Q,R)=d(R,S) is 5⋅4+5⋅411⋅10=40110=411. \dfrac{5\cdot 4+5\cdot 4}{11\cdot 10}=\dfrac{40}{110}=\dfrac{4}{11}. By the symmetry between QQ and S,S, P(d(Q,R)>d(R,S))=1−4112=722. \begin{gathered} P(d(Q,R)\gt d(R,S))\\ {}=\dfrac{1-\tfrac{4}{11}}{2}\\ {}=\dfrac{7}{22}. \end{gathered} Thus, the correct answer is A.
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Tagged: graph theory · basic probability · symmetry

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