#include<iostream>
#include<cstdio>
using namespace std;
int  main(){
    int t;
    cin>>t;
    while(t--){
        int n;
        cin>>n;
        int b[n];
        double c[n];
        double ans=0;
        for(int i=0;i<n;++i){
            cin>>b[i]>>c[i];
            ans=max(ans,(1-c[i])/(b[i]+(1-c[i])));
        }
        printf("%.5f\n",ans);
    }
    return 0;
}