#include <iostream>
#include <cstdio>
#include <algorithm>
using namespace std;
int a[303];
int main()
{
    //freopen("input.txt","r",stdin);
    int T;
    scanf("%d",&T);
    while(T--)
    {
        int n;
        scanf("%d",&n);
        for(int i=0;i<n;i++)
            cin>>a[i];
        sort(a,a+n);
        int ans=0;
        for(int i=0;i<n;i++)
        {
            int t=i+1;
            while(t<n && a[t]==a[i])t++;
            if(t!=n)ans+=n-t;
        }
        //cout<<ans<<endl;
        double p=(double)ans/(n*(n-1));
        printf("%.6lf\n",p);
    }
    return 0;
}