Practice problems from project Euler. Sometimes these are what I believe are the best way to solve it. Other times I have gone out of my way to avoid using a built in class such as 'Prime' and built things out myself. For example: inside largest_prime_factor I have used the Prime class to check if a number is prime, but I performed the algorithm using base Ruby. There is possibly a cleaner way to accomplish this using more of the Prime class, but I wanted the experience of building more of it out.