#include <iostream>
#include <cstdio>
#include <cstring>
using namespace std;

const int MAXN = 310;

int a[MAXN];

int main() {
    int t;
    scanf("%d", &t);
    while(t--) {
        int n, num;
        scanf("%d", &n);
        memset(a, 0, sizeof(a));
        for(int i = 1; i <= n; i++) {
            scanf("%d", &num);
            a[num]++;
        }
        for(int i = 1; i <= 300; i++) a[i] += a[i - 1];
        int total = 0;
        for(int i = 1; i <= 300; i++)
            if(a[i] - a[i - 1]) total += (a[i] - a[i - 1]) * (a[i - 1]);
        double ans = (double)total / (double)n / (double)(n - 1);
        printf("%.6lf\n", ans);
    }
    return 0;
}