This site also includes an unsolved problem category. You are given one dollar. You are told that your dollar will double for every time you toss a coin and it lands on heads. The first time you toss a tails the game is over and you keep what you have. For example if you toss three heads and then tails you would get one dollar times two times two times two, or eight dollars.
All problems provide an answer while some also provide an explanation of the solution.
Sources for these problems are other puzzle pages, math books, and the web page's author.