I was doing some old International Mathematical Olympiad (IMO) problems and one question went this way:
In a party with 1982 persons, among any group of 4, there is at least one person, who knows each of other three. What is the minimum number of people in the party who knows everyone else?
My knowledge of combinatorics is not working here. I'm stuck and completely blank, hope you guys can help me with it (You can use advance combinatorics but in a simple manner). Thanks.