#include<bits/stdc++.h>
signed main()
{
    int test;
    for ( scanf("%d",&test);test--; )
    {
        int n,b;double c,ans=0;scanf("%d",&n);
        for ( int i=1;i<=n;i++ ) scanf("%d%lf",&b,&c),ans=std::max(ans,(1-c)/(b+1-c));
        printf("%.5lf\n",ans);
    }
}