people visited an exhibition where paintings were displayed. Prove that it is possible to send some of the visitors to two rooms, with at least one visitor in each room, such that there is no painting that was liked by someone in one room but by nobody in the other, and there is no painting whose painter is personally known by someone in one room but by nobody in the other.
, 2026
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puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.