IMO Shortlist 1964 problem 3
Dodao/la:
arhiva2. travnja 2012. Seventeen people correspond by mail with one another-each one with all the rest. In their letters only three different topics are discussed. each pair of correspondents deals with only one of these topics. Prove that there are at least three people who write to each other about the same topic.
%V0
Seventeen people correspond by mail with one another-each one with all the rest. In their letters only three different topics are discussed. each pair of correspondents deals with only one of these topics. Prove that there are at least three people who write to each other about the same topic.
Izvor: Međunarodna matematička olimpijada, shortlist 1964
Školjka 
