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Found problems: 1

2024 Rioplatense Mathematical Olympiad, 4
There are 4 countries: Argentina, Brazil, Peru and Uruguay. Each country consists of 4 islands. There are bridges going back and forth between some of the 16 islands. Carlos noted that whenever he travels between some of the islands using the bridges, without using the same bridge twice, and ending in the island where he started his journey, he will necessarily visit at least one island of each country. Determine the maximum number of bridges there can be.
combinatorics countries rioplatense double counting