Multiples of 3 or 5

This is a programming version of Problem 1 from projecteuler.net.

Code

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#include <bits/stdc++.h>
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using namespace std;
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int main() {
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    int t;
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    cin >> t;
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    for (int a0 = 0; a0 < t; a0++) {
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        int n;
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        cin >> n; n--;
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        long long ans = 0;
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        long long a = n / 3;
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        a = (3 * a * (a + 1)) / 2;
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        ans += a;
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        a = n / 5;
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        ans += (a * (a + 1) * 5) / 2;
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        a = n / 15;
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        ans -= (15 * a * (a + 1)) / 2;
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        cout << ans << "\n";
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    }
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    return 0;
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}

Give it a try first

Attempt the problem on HackerRank — the code will still be here.