#include <bits/stdc++.h>

using namespace std;

int main()
{
    // freopen("input.txt", "r", stdin);
    // freopen("output.txt", "w", stdout);
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    int T;
    cin >> T;
    while(T--)
    {
        int n;
        cin >> n;
        double ans(0.0);
        while(n--)
        {
            double b, c;
            cin >> b >> c;
            ans = max(ans, (1 - c) / (b + 1 - c));
        }
        cout << fixed << setprecision(5) << ans << endl;
    }
    return 0;
}