Problem

Source: Tournament of Towns 2011 oral p4

Tags: combinatorics



A subset of a student group is called an ideal company if 1) in this subset, all girls are liked by all young men, 2) no one can be added to this subset without violating condition $1$. In a certain group, $9$ female students and $15$ students study. Warden of the group made a list of all kinds of ideal companies in this group. What is the largest number of companies on this list?