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

using namespace std;

typedef long long LL;
const int N = 1e3 + 7;
int n, a[N];

int work(int T = 0)
{
    memset(a, 0, sizeof(a));
    scanf("%d", &n);
    for(int i = 0, x; i < n; i ++)
        scanf("%d", &x), a[x]++;
    LL cnt = 0, tmp = 0;
    for(int i = 300; i > 0; i --)
        cnt += a[i] * (n-tmp-a[i]), tmp += a[i];
    printf("%.6f\n", 1.0*cnt/(n*(n-1)));
    return 0;
}

int main()
{
    int T = 0;
    scanf("%d", &T);
    for(int i = 1; i <= T; i ++)
        work(i);
    return 0;
}