KöMaL problem A, 2023

Combinatorics Difficulty 6.0 National Olympiad Prove it Hungary

$n$ people live in a town, and they are members of some clubs (residents can be members of more than one club). No matter how we choose some (but at least one) clubs, there is a resident of the town who is the member of an odd number of the chosen clubs. Prove that the number of clubs is at most $n$ .

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