Project Euler is a series of challenging mathematical/computer programming problems.
Each problem has been designed according to a "one-minute rule", which means that although it may take several hours to design a successful algorithm with more difficult problems, an efficient implementation will allow a solution to be obtained on a modestly powered computer in less than one minute.
More details can be found at https://projecteuler.net
These are my solutions to each problem.
If you don't want to spoil the fun, don't look!