Project Euler

I recently found an interesting site called Project Euler. This site attempts to provide a platform for the inquiring mind to delve into unfamiliar areas and learn new concepts in a fun and recreational context. This is being done by publishing different mathematical problems of varying difficulty.
I liked the concept so I thought about joining the site, but after taking a closer look on the problems I lost my motivation - most of the problems are meant to be solved using a computer. I just can't feel motivated to write a program in order to sum all the primes less than 2 million (this is problem number 10). However, if you like this type of problems this is clearly an excellent site. They have lots of problems of varying difficulty and new problems are constantly added.

For those who like my don't like using computer to solve problems there are problems that don't require a computer to solve - for example the first problem: "Add all the natural numbers below one thousand that are multiples of 3 or 5." This one is a very simple problem, so I guess it will be fine to post a hint to a solution. All you need to do is to sum the arithmetic progressions 3,6,9,..... and 5,10,15..... If you add the sums you will get the result, but the numbers that are multiples of both 3 and 5 will be counted twice.
I also really liked problem number 205. It is pretty easy to solve, but it requires some thinking and there is no need for a computer.