Find the sum of all the multiples of 3 or 5 below 1000

Question

If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9. The sum of these multiples is 23. I have the following code but the answer do

Answer 1

Perhaps you should do

sum += i // or sum = sum + i

instead of

sum = sum + 1

Additionally, be careful when printing long unsigned ints with printf. I guess the right specifier is %lu.