Whilst tag-surfing I discovered Project Euler, which looks like fun for people who enjoy mathematical puzzles. Here’s what they say about themselves:
What is Project Euler?
Project Euler is a series of challenging mathematical/computer programming problems that will require more than just mathematical insights to solve. Although mathematics will help you arrive at elegant and efficient methods, the use of a computer and programming skills will be required to solve most problems.
The motivation for starting Project Euler, and its continuation, is to provide a platform for the inquiring mind to delve into unfamiliar areas and learn new concepts in a fun and recreational context.”
After you register and log in you can post an answer to the questions, and they will tell you if it is right or not.