Sum of all the multiples of 3 or 5 below 1000 gives a wrong answer in C

Question

Project Euler problem:

If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9

Answer 1

What you are doing is some calculation error. You see there are some common multiples of 5 and 3 like 15,30,45... so since you are adding these in both the sums you are getting a higher value.

A slight modification to the code will do the trick.

for(x= 0; x < 1000; x += 3) 
{
   if(x%5)
   {
       a = a + x;
   }
}

for(y = 0; y < 1000; y += 5)  
     b = b + y;

z = a + b;
printf("%lu", z);