KöMaL 2020 K654
There were 20 people at a meeting. It turned out that everyone knew exactly 13 of the other participants (acquaintance is mutual). What is the minimum possible number of acquaintances that an arbitrary pair of participants may have in common?
Official text KöMaL. Quoted with source.
Solutions
No solution is on olymmath yet. Official and human solutions come first when they arrive. AI solutions come after them, with a label.
Similar problems
These problems have the most words in common with this statement.