Q: Project Euler #1 - C#
Hint:

$$
\sum_{i=1}^ni = \frac{n (n+1)}{2}
$$

With this you can compute the sum of 1 to n without looping. Can you figure out a way to do this for all digits with increments of 3? And 5?

The last thing to consider is that with this method, you are counting all digits divisible by 15 double, since they are both divisible by 3 and 5.