IMO Shortlist 1970 problem 12
Given
coplanar points, no three collinear, prove that at most
of the triangles formed by the points have all angles acute.
%V0
Given $100$ coplanar points, no three collinear, prove that at most $70\%$ of the triangles formed by the points have all angles acute.
Source: Međunarodna matematička olimpijada, shortlist 1970