IMO Shortlist 1990 problem 3
Avg:
Avg:
Let
and consider a set
of
distinct points on a circle. Suppose that exactly
of these points are to be colored black. Such a coloring is good if there is at least one pair of black points such that the interior of one of the arcs between them contains exactly
points from
. Find the smallest value of
so that every such coloring of
points of
is good.









Izvor: Međunarodna matematička olimpijada, shortlist 1990