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One of the [[classical combinatorial problems]].

In 1850 the Revd T.P. Kirkman published in ''The Lady's and Gentleman's Diary'' for 1850 the problem ''Fifteen young ladies in a school walk out three abreast for seven days in succession: it is required to arrange them daily, so that not two shall walk twice abreast''.  He published his solution in the 1851 edition.  

In modern terms this asks for a resolvable [[Steiner triple system]], otherwise known as a [[Kirkman triple system]], on 15 points.  

====References====
* Robin Wilson, "Combinatorics: A Very Short Introduction", Oxford University Press (2016) {{ZBL|1366.05001}}