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

#define Int register int
#define MAXN 105

template <typename T> inline void read (T &t){t = 0;char c = getchar();int f = 1;while (c < '0' || c > '9'){if (c == '-') f = -f;c = getchar();}while (c >= '0' && c <= '9'){t = (t << 3) + (t << 1) + c - '0';c = getchar();} t *= f;}
template <typename T,typename ... Args> inline void read (T &t,Args&... args){read (t);read (args...);}
template <typename T> inline void write (T x){if (x < 0){x = -x;putchar ('-');}if (x > 9) write (x / 10);putchar (x % 10 + '0');}

int n;

signed main(){
    int t;read (t);
    while (t --){
        read (n);double maxn = 0,a,c;
        for (Int i = 1;i <= n;++ i) scanf ("%lf%lf",&a,&c),maxn = max (maxn,(1 - c) / (a + 1 - c));
        printf ("%.5f\n",maxn);    
    }
    return 0;
}