112 groups of 11 members each with one common in every two groups
Source: Greece JBMO TST 2015 p4
April 29, 2019
combinatoricsset theorySets
Problem Statement
Pupils of a school are divided into groups, of members each.
Any two groups have exactly one common pupil. Prove that:
a) there is a pupil that belongs to at least groups.
b) there is a pupil that belongs to all the groups.