If two friends are close friends, they send private messages outside the group.

[xiang1805]

"There is a chat group of 6 friends: Andy, Benny, Cindy, Danny, Elisa and Fabio. They are close to the group but not necessarily everyone is close to everyone.
It is known that if two friends are close friends, they send private message to each other outside the group.

Question:
It is known that everyone has the same number of close friends. How many ways that this situation can happen?"

 

[stapel]

There is a chat group of 6 friends: Andy, Benny, Cindy, Danny, Elisa and Fabio. They are close to the group but not necessarily everyone is close to everyone. It is known that if two friends are close friends, they send private message to each other outside the group.

Question: It is known that everyone has the same number of close friends. In how many ways can this situation happen?

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