The art of Problem Solving

I've dedicated some room on this site especially for those who are intrigued by challenging puzzles and riddles. Undoubtedly, every person has come along some tough problems. However, in the eyes of mathematicians and math-enthousiasts, such hard problems suddenly obtain a beautiful aura when the solution is either creative, elegant and/or unexpectedly short. I hope you enjoy some of the puzzles I've listed here, as I did for sure.

Every tenth problem could be considered a challenge, and every title in italics is a problem for which I didn't find a solution yet. Care to help me out?

If you want to contribute to a solution or suggest your own favorite puzzle, please don't hesitate to contact me!