#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>
#include <iostream>
#include <iomanip>

using namespace std;
const int maxn=310;
int a[maxn];
int vis[maxn];

int main()
{
    int T,i,j,n,temp,cnt;
    scanf("%d",&T);
    while (T--)
    {
        scanf("%d",&n);
        memset(a,0,sizeof(a));
        
        cnt=0;
        for (i=0;i<n;i++)
        {
            scanf("%d",&temp);
            if (a[temp]==0)
            {
                vis[cnt++]=temp;
            }
            a[temp]++;
        }
        
        sort(vis,vis+cnt);
        double fenmu=0,fenzi=0;
        for (i=0;i<cnt;i++)
        {
            for (j=i+1;j<cnt;j++)
            {
                fenzi+=a[vis[i]]*a[vis[j]];
            }
        }
        fenmu=n*(n-1);
        printf("%.6lf\n",fenzi/fenmu);
    }
}