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

const int N = 333;

int n;
int a[N];

int main()
{
    int t;
    scanf("%d", &t);
    while(t--) {
        scanf("%d", &n);
        for(int i = 0; i < n; ++i) scanf("%d", a+i);
        sort(a, a+n);
        double s = 0.0;
        for(int i = 0; i < n; ++i) {
            int x = lower_bound(a, a + n, a[i]) - a;
//        printf("x == %d\n", x);
            s += 1. * x ;
        }
        printf("%.6f\n", s / (n * (n-1)) );
    }
    return 0;
}