IMO 1972 LL MON24
The diagonals of a convex 18-gon are colored in 5 different
IMO 1972 LL MON24
Origin: MON
Problem
The diagonals of a convex 18-gon are colored in 5 different colors, each color appearing on an equal number of diagonals. The diag- onals of one color are numbered 1, 2, . . .. One randomly chooses one-fifth of all the diagonals. Find the number of possibilities for which among the chosen diagonals there exist exactly n pairs of diagonals of the same color and with fixed indices i, j.