Multiples of 3 or 5

_{This problem is a programming version of Problem 1 from projecteuler.net}

Code

C++

#include <bits/stdc++.h>
using namespace std;

int main() {
    int t;
    cin >> t;
    for (int a0 = 0; a0 < t; a0++) {
        int n;
        cin >> n; n--;

        long long ans = 0;
        long long a = n / 3;

        a = (3 * a * (a + 1)) / 2;
        ans += a;

        a = n / 5;
        ans += (a * (a + 1) * 5) / 2;

        a = n / 15;
        ans -= (15 * a * (a + 1)) / 2;

        cout << ans << "\n";
    }
    return 0;
}