At a meeting of people, each person exchanges greetings with exactly others. For any two people, the number who exchange greetings with both is the same. How many people are at the meeting?
%V0
At a meeting of $12k$ people, each person exchanges greetings with exactly $3k+6$ others. For any two people, the number who exchange greetings with both is the same. How many people are at the meeting?
Izvor: Međunarodna matematička olimpijada, shortlist 1995
Školjka 
people, each person exchanges greetings with exactly 
others. For any two people, the number who exchange greetings with both is the same. How many people are at the meeting?