MultiplesOf3And5.c

#include <math.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <assert.h>
#include <limits.h>
#include <stdbool.h>

int main(){
    int t; 
    scanf("%d",&t);
    for(int a0 = 0; a0 < t; a0++){
        int n; 
        scanf("%d",&n);

        long a = (n - 1) / 3;
        long b = (n - 1) / 5;
        long d = (n - 1) / 15;

        long sum3 = 3 * a * (a + 1) / 2;
        long sum5 = 5 * b * (b + 1) / 2;
        long sum15 = 15 * d * (d + 1) / 2;
        long c = sum3 + sum5 - sum15;
        printf("%ld\n",c);
    }
    return 0;
}