#include <bits/stdc++.h>
#define ll long long
using namespace std;
int main() {
    int T, n;
    scanf("%d", &T);
    while(T--) {
        scanf("%d", &n);
        double ans = 0;
        for(int i = 0, b; i < n; i++) {
            double c;
            scanf("%d%lf", &b, &c);
            ans = max(ans, (1 - c) / (b + 1 - c));
        }
        printf("%.5f\n", ans);
    }
    return 0;
}