Euler Challenge 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.
Using your head is permitted Monthly math riddle. Some quite probabilistic in nature (at least one was suggested by Omer angel))
The Python challenge The Python Challenge is a set of riddles that require a little bit of Python programming to be solved. The solutions are entered by changing the address of the page (URL). You get used to the idea pretty fast after solving the first few levels.
Oxford interview questions Self-Explanatory.