The diagonals of a quadrilateral
A quadrilateral of any shape whatever is divided by a transversal into two quadrilaterals. The diagonals of the original figure and those of the two resulting (smaller) figures are then drawn. Show that their three points of intersection are collinear. (American Mathematical Monthly [Vol. 22, No. 5 (May 1915),pp. 165-166]
(solved by Anna Margaret Mullikin (March 7, 1893 - August 24, 1975)