Problem

Source: 2021 Saudi Arabia Training Lists p27 https://artofproblemsolving.com/community/c2758131_2021_saudi_arabia_training_tests

Tags: combinatorics



Each of $N$ people have chosen some $5$ elements from a $23$-element set so that any two people share at most $3$ chosen elements. Does this mean that $N \le 2020$? Answer the same question with $25$ instead of $23$.