1539A - Contest Start - CodeForces Solution

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

typedef long long int ll;
typedef pair<ll, ll> pl;

#define N 100010

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    int t; cin >> t;
    while(t--) {
        ll n, x, t; cin >> n >> x >> t;
        ll d = t / x;
        if(d > n - 1) {
            cout << ((n - 1) * n) / 2 << "\n";
        } else {
            ll nn = max(0LL, n - d);
            cout << (nn * d + ((d - 1) * d) / 2) << "\n";
        }
    }
}