FAVORITE MATH PROBLEM
I like this problem because it requires the person solving it to use knowledge from several different areas of mathematics. None of the skills required are particularly difficult; a student who has completed Calculus I should have all the knowledge needed to obtain the solution. The challenge lies in knowing which skills to use at what point, and how to use them together to accomplish the goal. Learning to do this can benefit both students and mathematical researchers alike.
Modified from the problem found here.
Modified from the problem found here.