#include <bits/stdc++.h>
using namespace std;
void solve() {
    double ans = 0;
    int n;
    cin >> n;
    while (n--) {
        int b;
        double c;
        cin >> b >> c;
        ans = max(ans, (1 - c) / (b + 1 - c));
    }
    printf("%.5lf\n", ans);
}
int main() {
    int T;
    cin >> T;
    while (T--) {
        solve();
    }
    return 0;
}