In a school there are n classes and k student. We know that in this school every two students have attended exactly in one common class. Also due to smallness of school each class has less than k students. If k−1 is not a perfect square, prove that there exist a student that has attended in at least k classes.Proposed by Mohammad Moshtaghi Far, Kian Shamsaie Rated 4 graph theorycombinatorics