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OMM 2020 Problem 5

    A four-element set {a,b,c,d}\{a, b, c, d\} of positive integers is called good if there are two of them such that their product is a mutiple of the greatest common divisor of the remaining two. For example, the set {2,4,6,8}\{2, 4, 6, 8\} is good since the greatest common divisor of 22 and 66 is 22 , and it divides 4×8=324\times 8=32 .

    Find the greatest possible value of nn , such that any four-element set with elements less than or equal to nn is good.

    Proposed by Victor and Isaías de la Fuente

    Dataset text AI-MO AoPS. Quoted with source.

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