#include<iostream>
#include<cstdio>
using namespace std;
int main()
{
    int T;
    cin>>T;
    while(T--)
    {    
        double maxs=-1,a,b;
        int n;
        cin>>n;
        for(int i=1;i<=n;i++)
        {
            cin>>a>>b;
            double cnt=(1.0-b)/(a+(1.0-b));
            maxs=max(maxs,cnt);
        }
        printf("%.5lf\n",maxs);
    }
}