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

int main()
{
    int T,n;scanf("%d",&T);
    while(T--)
    {
        scanf("%d",&n);
        double ans=0;
        for(int i=1;i<=n;i++)
        {
            int b;double c;
            scanf("%d%lf",&b,&c);
            ans=max(ans,(1-c)/(b+1-c));
        }
        printf("%.5lf\n",ans);
    }
    return 0;
}