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Daniyar's Friends

Author: mathforces
Problem has been solved: 27 times

Русский язык | English Language

Once upon a time, Daniyar and five of his friends were in the MathForces community. One day he brought them together, and some of them managed to get to know each other. The next day after the meeting, Daniyar realized that if two people do not know each other, then they (among the remaining three people) do not have another person whom they both know. How many different situations are there in which this is possible?

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