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Found problems: 1

2014 Bosnia And Herzegovina - Regional Olympiad, 4
At the beginning of school year in one of the first grade classes: $i)$ every student had exatly $20$ acquaintances $ii)$ every two students knowing each other had exactly $13$ mutual acquaintances $iii)$ every two students not knowing each other had exactly $12$ mutual acquaintances Find number of students in this class
acquaintance combinatorics