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Why Sebastian and Oma left the Party

Here is the solution. Actually one can easily solve the problem with graph theory.

First I will give a situation where Sebastian and Oma stay and wait if the buffet will be opened, because there is a possible solution (also if the buffet might stay close: One person might know noone else for example.)

Lets have 5 persons and each should know two other persons

One can see that we have 5*2 = 10 edges. Since A knows B if B knows A, we also have 10 / 2 pairs of edges.
Generally we have persons * knownPersons edges with (persons* knownPersons / 2 ) pairs.

Lets apply this to our case: 15 people and each one knows 7 people. So we have 105 edges that occur as pairs. ..... No an odd number of edges cant occur as pairs. So the buffet will never be opened.
Sebastian found out that and left with Oma