This site is a compilation of interesting mathematical brain teasers.
A few disclaimers:
I try to give credit to the source of the brain teaser on the question page. If you think I didn’t give credit where credit is due, just email me and I’ll fix it.
Difficulty ratings are personal. They reflect how hard I found the problem. If I haven’t solved the problem yet, it’s obviously just a guess.
If the question is “Solved”, that doesn’t necessarily mean I solved it by myself. It just means that I’ve included a solution.
I post code in some of the solutions. It’s generally terribly written code, but for one-off math problems, I’d rather not spend the time to clean it up.