#include <algorithm>
#include <iostream>
#include <sstream>
#include <cstring>
#include <cstdlib>
#include <string>
#include <vector>
#include <cstdio>
#include <cmath>
#include <queue>
#include <stack>
#include <map>
#include <set>
using namespace std;
#define INF 0x3f3f3f3
const int N=150005;
const int mod=1e9+7;

int s[305],l[305];

int main() {
    int n,t;
    cin>>t;
    while (t--) {
        cin>>n;
        for (int i=1; i<=n; i++) {
            scanf("%d",&s[i]);
        }
        memset(l, 0, sizeof(l));
        int sum=0;
        sort(s+1, s+n+1);
        for (int i=2; i<=n; i++) {
            for (int j=1; j<i; j++) {
                if (s[i]>s[i-j]) {
                    l[i]=i-j;
                    break;
                }
            }
        }
        for (int i=1; i<=n; i++) {
            sum+=l[i];
        }
        double x=sum*1.0/(n*(n-1)*1.0);
        printf("%.6lf\n",x);
    }
    return 0;
}