Let X be a finite set of points on the plane such that each point in X is colored with red or blue and there is no line having all points in X. Prove that there is a line L having at least two points of X such that all points in L∩X have the same color.
The best solution was submitted by Minjae Park (박민재), 한국과학영재학교 (KSA). Congratulations!
Here is his Solution of Problem 2010-20.