This is a solution to Reverend Kirkman's schoolgirls problem in
combinatorics.
(See that writeup for more details.) If the 15 schoolgirls are denoted
by the letters a-o then a solution is given by:
Day 1 Day 2 Day 3 Day 4 Day 5 Day 6 Day 7
abc ade afg ahi ajk alm ano
dhl bik bhj beg cdf bef bdg
ejn cmo cln cmn blo cij chk
fio fhn dim djo ehm dkn eil
gkm gjl eko fkl gin gho fjm