#include <stdio.h>
 
int sum_of_series(int limit, int difference)
{
    int n = 1 + limit / difference; // integer math gives number of terms including term 0
    return ((n * (n - 1)) / 2) * difference;
}
 
int main(void){
 
    int sum_of_series_3 = sum_of_series(999,3);
    int sum_of_series_5 = sum_of_series(999,5);
    int sum_of_series_15 = sum_of_series(999,15);
 
    int answer = sum_of_series_3 + sum_of_series_5 - sum_of_series_15;    
 
    printf("sum of all the multiples of 3 or 5 below 1000\n");
    printf("answer = %d  3s=%d 5s=%d 15s=%d\n",answer, sum_of_series_3, sum_of_series_5, sum_of_series_15);
    return 0;
}

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