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

int main()
{
    int T;
    cin >> T;

    while(T --)
    {
        int n;
        cin >> n;

        double ans = 0.0;
        for(int i = 0; i < n; i ++)
        {
            double b, c;
            scanf("%lf%lf", &b, &c);

            c = 1.0 - c;
            ans = max(ans, c / (b + c));
        }

        printf("%.5lf\n", ans);
    }
    return 0;
}