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Two random subsets of five elements overlapping

Question

Two subsets AA and BB of {1,2,3,4,5}\{1,2,3,4,5\} are chosen independently and uniformly from all 3232 subsets. What is the probability that they share at least one element?

Short answer

781/1024781/1024. Each element is independently in both sets with probability 14\tfrac14. The sets are disjoint when no element is in both, which has probability (3/4)5=243/1024(3/4)^5=243/1024, so the answer is 1−24310241-\tfrac{243}{1024}.

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