(Based on this puzzle by Gamow. The answer there might give you a hint (so don't look yet!))
At a fancy party in Café la Tour, everybody is friends with exactly 14 of the other people present.
Whenever two people at the party are friends, they have exactly 6 friends (present at the party) in common.
Whenever two people at the party are not friends, they have exactly 2 friends (present at the party) in common.
(Oh, and half of the people at the party came dressed in white, and the other half was dressed in black. How stylish!)
Question: How many persons are at the party?
(Note: Friendships are always mutual. Nobody is their own friend. There is at least one person at the party.)