 Kirkman's schoolgirl problem

Kirkman's schoolgirl problem is a classic problem in combinatorial design and graph theory, posed by the mathematician Thomas Kirkman in 1850. The problem states the following: There are 15 schoolgirls who take part in a walking exercise. Each day, they walk in groups of three, and the condition is that each girl must walk with every other girl exactly once over a series of days. The challenge is to arrange these walks in such a way that the requirement is met.