Question 100
Several people gathered at a party. Each two of them who don't know each other have exactly two common acquaintances (if a person $A$ knows a person $B$, then necessarily the person $B$ knows the person $A$). Mykhailyk and Vitalyk know each other, but have no common acquaintances.
a) Prove that Mykhailyk and Vitalyk have the same number of acquaintances on the party.
b) Can such a situation happen on a party with exactly 6 people?
a) Prove that Mykhailyk and Vitalyk have the same number of acquaintances on the party.
b) Can such a situation happen on a party with exactly 6 people?