/* ***********************************************
Author        :½aóÂ
Created Time  :2016/3/19 17:38:23
File Name     :F:\3ìDòÔ′\HDU\BC76\5645.cpp
************************************************ */

#include <stdio.h>
#include <string.h>
#include <iostream>
#include <algorithm>
#include <vector>
#include <queue>
#include <set>
#include <map>
#include <string>
#include <math.h>
#include <stdlib.h>
#include <time.h>
using namespace std;

int main()
{
    //freopen("in.txt","r",stdin);
    //freopen("out.txt","w",stdout);
    int t,a[305],n;
    scanf("%d",&t);
    while(t--)
    {
        scanf("%d",&n);
        for(int i=0;i<n;i++)
        {
            scanf("%d",&a[i]);
        }
        int ans=0;
        for(int i=0;i<n;i++)
        {
            for(int j=i+1;j<n;j++)
            {
                if(a[i]!=a[j])
                    ans++;
            }
        }
        double one=ans*1.0/(n*(n-1))*1.0;
        printf("%.6lf\n",one);
    }
    return 0;
}