#include <bits/stdc++.h>
using namespace std;
const int maxn = 262144;
struct comp{
    double x, y;
    comp(double _x = 0, double _y = 0){
        x = _x;
        y = _y;
    }
    friend comp operator * (const comp &a, const comp &b){
        return comp(a.x * b.x - a.y * b.y, a.x * b.y + a.y * b.x);
    }
    friend comp operator + (const comp &a, const comp &b){
        return comp(a.x + b.x, a.y + b.y);
    }
    friend comp operator - (const comp &a, const comp &b){
        return comp(a.x - b.x, a.y - b.y);
    }
}f[maxn];
int rev[maxn];
void dft(comp A[], int len, int kind){
    for(int i = 0; i < len; i++){
        if(i < rev[i]){
            swap(A[i], A[rev[i]]);
        }
    }
    for(int i = 1; i < len; i <<= 1){
        comp wn(cos(acos(-1.0) / i), kind * sin(acos(-1.0) / i));
        for(int j = 0; j < len; j += (i << 1)){
            comp tmp(1, 0);
            for(int k = 0; k < i; k++){
                comp s = A[j + k], t = tmp * A[i + j + k];
                A[j + k] = s + t;
                A[i + j + k] = s - t;
                tmp = tmp * wn;
            }
        }
    }
    if(kind == -1) for(int i = 0; i < len; i++) A[i].x /= len;
}
void init(int &len){
    int L = 0;
    for(len = 1; len < maxn; len <<= 1, L++);
    for(int i = 0; i < len; i++){
        rev[i] = rev[i >> 1] >> 1 | (i & 1) << L - 1;
    }    
}
int a[maxn], val[maxn] = {0};
long long num[maxn];
int main(){
    int t;
    scanf("%d", &t);
    int len;
    init(len);
    while(t--){    
        int n;
        scanf("%d", &n);
        for(int i = 1; i <= n; i++){
            scanf("%d", a + i);
            val[a[i]]++;
        }
        for(int i = 0; i < len; i++){
            f[i].x = val[i];
        }
        dft(f, len, 1);
        for(int i = 0; i < len; i++) f[i] = f[i] * f[i];
        dft(f, len, -1);
        for(int i = 1; i < len; i++) num[i] = (long long)(f[i].x + 0.5);
        for(int i = 1; i <= n; i++){
            num[a[i] << 1]--;
        }
        for(int i = 1; i < len; i++) num[i] = num[i] >> 1;
        num[0] = 0;
        for(int i = 1; i < len; i++) num[i] += num[i - 1]; 
        long long ans = 0;
        for(int i = 1; i <= n; i++){
            ans += num[a[i]];
        }
        printf("%.7lf\n", 1 - 6.0 * ans / n / (n - 1) / (n - 2));
        for(int i = 1; i <= n; i++) val[a[i]]--;
        for(int i = 0; i < len; i++) f[i] = comp();
    }
    return 0;
}