#include <iostream>
#include <cstdio>
#include <cmath>
#include <algorithm>
#include <cstring>
using namespace std;

const double PI = acos(-1.0);

struct Complex {
    double x, y;
    Complex(double _x = 0.0, double _y = 0.0) {
        x = _x;
        y = _y;
    }
    Complex operator -(const Complex &b)const {
        return Complex(x-b.x, y-b.y);
    }
    Complex operator +(const Complex &b)const {
        return Complex(x+b.x, y+b.y);
    }
    Complex operator *(const Complex &b)const {
        return Complex(x*b.x-y*b.y, x*b.y+y*b.x);
    }
};
void change(Complex y[], int len) {
    int i, j, k;
//    cout << len << endl;
//    cout << "-----" << endl;
    for(i = 1, j = len/2; i < len-1; i++) {
        if( i < j ) swap(y[i], y[j]);
        k = len / 2;
        while( j >= k ) {
            j -= k;
            k /= 2;
//            cout << "----" << endl;
//            cout << j << " " << k << endl;
        }
        if( j < k ) j += k;
//        cout << "==" << endl;
        
    }
//    cout << "-----" << endl;
//    cout << "-------------" << endl;
}

void fft(Complex y[], int len, int on) {
    
    change(y, len);
//    cout << "----" << endl;
    for(int h = 2; h <= len; h <<= 1) {
        Complex wn(cos(-on*2*PI/h), sin(-on*2*PI/h));
        for(int j = 0; j < len; j+=h) {
            Complex w(1,0);
            for(int k = j; k < j+h/2; k++) {
                Complex u = y[k];
                Complex t = w*y[k+h/2];
                y[k] = u+t;
                y[k+h/2] = u-t;
                w = w*wn;
            }
        }
    }
    if( on == -1 ) {
        for( int i = 0; i < len; i++ ) {
            y[i].x /= len;
        }
    }
}

const int MAXN = 400040;
Complex x1[MAXN];
int a[MAXN/4];
long long num[MAXN];
long long sum[MAXN];

int main() {
    int t;
    int n;
    scanf( "%d", &t);
    while( t-- ) {
        scanf( "%d", &n );
        memset( num, 0, sizeof( num ) );
        for( int i = 0; i < n; i++ ) {
            scanf( "%d", &a[i] );
            num[a[i]]++; // 记录a[i]出现的次数
        }
//        cout << "=====" << endl;
        sort(a, a+n);
        int len1 = a[n-1] + 1; // len 为最长边的界
        int len = 1;
        while( len < 2 * len1 ) len <<= 1;
        for( int i = 0; i < len1; i++ ) {
            x1[i] = Complex(num[i], 0);
        }
        for( int i = len1; i < len; i++ ) {
            x1[i] = Complex(0,0);
        }
//        cout << "======" << endl;
        fft(x1, len, 1);
//        cout << "-----" << endl;
        for( int i = 0; i < len; i++ ) {
            x1[i] = x1[i] * x1[i];
        }
        fft(x1, len, -1);
//        cout << "-----" << endl;
        for( int i = 0; i < len; i++ ) {
            num[i] = (long long)(x1[i].x + 0.5);
        }
        len = 2 * a[n-1]; // 教程中 界为 2n 必须由 2n 个点来确定
        for(int i  = 0; i < n; i++ ) {
            num[a[i]+a[i]]--;   // 去重
        }
        for(int i = 1; i <= len; i++ ) {
            num[i] /= 2;   // 卷积后求得是排列数， 组合数除以2
        }
        sum[0] = 0;
        for( int i = 1; i <= len; i++ ) {
            sum[i] = sum[i-1] + num[i];
        }
        long long cnt = 0;
        for( int i = 0; i < n; i++ ) {
            cnt += sum[len] - sum[a[i]];  // 长度和 大于 a[i] 的个数
            cnt -= (long long)(n-1-i)*i; // 减去一大一小
            cnt -= (n-1); // 减去包括本身的
            cnt -= (long long)(n-1-i)*(n-2-i)/2; // 减去两个大的组合数
        }
        long long tot = (long long)n * (n-1) * (n-2) / 6;
//        cout << cnt << "===" << tot << endl;
        printf( "%.7lf\n", (double)cnt / tot );
    }
    return 0;
}