[reuben_tate]

Decided to start doing this again. Going to start from the beginning because I know when I first started my solutions were crap. I used my favorite programming language for this one:

Some actual code:

/*
* We use the fact that 3+6+9+12+... = 3(1+2+3+..) from which we can
* use the n(n+1)/2 formula for the sum of the first n positve integers
*/
       int a = 999/3; //number of multiples of 3 less than 1000
       int b = 999/5; //number of multiples of 5 less than 1000
       int c = 999/15; //number of multiples of 15 less than 1000
       System.out.println(3*(a)*(a+1)/2+5*(b)*(b+1)/2-15*(c)*(c+1)/2);