#include<iostream>
#include<cstring>
using namespace std;
const int N=1e5+5;
int b[N];
double c[N];
int main()
{
    int T;cin>>T;
    while(T--)
    {
        
        int n;cin>>n;
        for(int i=1;i<=n;i++)
        {
            cin>>b[i]>>c[i];
        }
        double sum=0.0;
        for(int i=1;i<=n;i++)
        {
            double ter=1.0-c[i];
            double tem=b[i]+ter;
            sum=max(sum,ter/tem);
        }
        printf("%.5lf\n",sum);
    }
}