I am a monster & very sorry. Also, this is unlikely to be correct.

fizzbuzz_with_variables.c

#include <stdio.h>

int main ()
{
     int three = 3, five = 5;
     int i;
     for (i = 0; i < 20; i++) {
          if (i != three && i != five) {
               printf("%d\n", i);
               continue;
          }
          if (i == three) {
               three += 3;
               printf("Fizz");
          }
          if (i == five) {
               five += 5;
               printf("Buzz");
          }
          putchar('\n');
     }
     return 0;
}