#include <iostream>

#include <cstdio>

#include <algorithm>

#include <cstring>

#include <cmath>

using namespace std;

typedef long long ll;

typedef pair<double,double>ppd;

const double PI = acos(-1.);

const int MAXL = (1 << 18)+100;

int a[MAXL],b[MAXL];

ll c[MAXL];

ppd A[MAXL],B[MAXL],C[MAXL],T[MAXL];

int id,ln;

void fill0(int m,int d,int *s,ppd *P)

{

    if (m == ln)

        P[d] = make_pair(s[id++],0.0);

    else

    {

        fill0(m<<1,d,s,P);

        fill0(m<<1,d+m,s,P);

    }

}

void fill1(int m,int d,ppd *C,ppd *P)

{

    if (m == ln) P[d] = C[id++];

    else

    {

        fill1(m<<1,d,C,P);

        fill1(m<<1,d+m,C,P);

    }

}

void fft(double oper,ppd *P)

{

    for (int d = 0;(1 << d) < ln;++d)

    {

        int m = (1 << d);

        double p0 = PI / m * oper;

        double sinp0 = sin(p0);

        double cosp0 = cos(p0);

        for (int i = 0;i < ln;i += (m << 1))

        {

            double sinp = 0;

            double cosp = 1;

            for (int j = 0;j < m;++j)

            {

                double ta = cosp * P[i+j+m].first - sinp*P[i+j+m].second;

                double tb = cosp * P[i+j+m].second + sinp * P[i+j+m].first;

                P[i+j+m].first = P[i+j].first - ta;

                P[i+j+m].second = P[i+j].second - tb;

                P[i+j].first += ta;

                P[i+j].second += tb;

                double tsinp = sinp;

                sinp = sinp * cosp0 + cosp * sinp0;

                cosp = cosp * cosp0 -tsinp * sinp0;

            }

        }

    }

}

void polyMul(int *a,int *b,ll *c,int n)

{

    ln=(1<<18);

    while((ln>>2)>=n) ln=ln>>1;

    id = 0;

    fill0(1,0,a,A);

    fft(1.0,A);

    id = 0;

    fill0(1,0,b,B);

    fft(1.0,B);

    for (int i = 0;i < ln;++i)

    {

        C[i].first = A[i].first * B[i].first - A[i].second * B[i].second;

        C[i].second = A[i].first * B[i].second + A[i].second * B[i].first;

    }

    id = 0;

    fill1(1,0,C,T);

    fft(-1.0,T);

    for (int i = 0;i < ln;++i)

        c[i] = ll(T[i].first / ln + 0.1);

}

int main()

{

    //freopen("data.in","r",stdin);

    //freopen("1010.in","r",stdin);

    //freopen("my.out","w",stdout);

    int T;

    scanf("%d",&T);

    while(T--)

    {

        int m;

        scanf("%d",&m);

        memset(a,0,sizeof(a));

        memset(b,0,sizeof(b));

        int n=0;

        for(int i=0;i<m;++i)

        {

            int tmp=0;

            scanf("%d",&tmp);

            n=max(n,tmp);

            ++a[tmp];

            ++b[tmp];

        }

        ++n;

        polyMul(a,b,c,n);

        for(int i=0;i<n;++i)

        if(a[i])

        c[i+i]-=a[i];

        for(int i=0;i<ln;++i)

        c[i]=c[i]>>1;

        double tmp=0.0;

        double t=0;

        for(int i=0;i<n;++i)

        {

            t+=c[i];

            tmp+=(a[i]*t);

        }

        double sum=(1.0*m*(m-1)*(m-2)/2/3);

        printf("%.7lf\n",(sum-tmp)/sum);

    }

    return 0;

}