#include <iostream>
/*每天在CF上刷A,B,C题各一道*/
#include <cstdio>
#include <cstring>
#include <cmath>
#include <cstdlib>
#include <algorithm>
#include <queue>
#include <stack>
#include <map>
#include <set>
#include <vector>
#include <string>
#define INF 0x3f3f3f3f
#define eps 1e-8
#define SIZE (300+10)
#define Ri(a) scanf("%d", &a)
#define Rl(a) scanf("%I64d", &a)
#define Rf(a) scanf("%lf", &a)
#define Rs(a) scanf("%s", a)
#define Pi(a) printf("%d\n", (a))
#define Pf(a) printf("%lf\n", (a))
#define Pl(a) printf("%I64d\n", (a))
#define Ps(a) printf("%s\n", (a))
#define W(a) while((a)--)
#define CLR(a, b) memset(a, (b), sizeof(a))
#define MOD 1000000007
#define ll __int64
#define lson l, mid, rt<<1
#define rson mid+1, r, rt<<1|1
#define PI acos(-1.0)
using namespace std;
int n;
int vis[310];

int main()
{
    int t;
    Ri(t);
    while(t--)
    {
        int x;
        CLR(vis,0);
        Ri(n);
        for(int i = 0; i < n; i++)
        {
            scanf("%d",&x);
            vis[x]++;
        }
        int ans = 0;
        for(int i = 1; i < 301; i++)
            for(int j = i+1; j < 301; j++)
            {

                ans += vis[i]*vis[j];

            }

            printf("%.6lf\n",((double)ans/(double)(n*(n-1))));

    }
    return 0;
}