#include <iostream>

#include <cstdio>

#include <cstring>

#include <algorithm>

#include <cmath>

#define N 100010

using namespace std; typedef double db;

const db pi=acos(-1); typedef long long ll;

inline char nc() {static char *p1,*p2,buf[100000]; return (p1==p2)&&(p2=(p1=buf)+fread(buf,1,100000,stdin),p1==p2)?EOF:*p1++;}

int rd() {int x=0; char c=nc(); while(!isdigit(c)) c=nc(); while(isdigit(c)) x=(x<<3)+(x<<1)+(c^48),c=nc(); return x;}

struct cp

{

	db x,y;

	cp() {x=y=0;}

	cp(db x_,db y_) {x=x_,y=y_;}

	cp operator + (const cp &a) const {return cp(x+a.x,y+a.y);}

	cp operator - (const cp &a) const {return cp(x-a.x,y-a.y);}

	cp operator * (const cp &a) const {return cp(x*a.x-y*a.y,x*a.y+y*a.x);}

}a[N<<2];

int n,maxx,b[N];ll num[100005];

void fft(cp *a,int len,int flg)

{

	int i,j,k,t;

	cp w,wn,tmp;

	for(i=k=0;i<len;i++)

	{

		if(i>k) swap(a[k],a[i]);

		for(j=len>>1;(k^=j)<j;j>>=1);

	}

	for(k=2;k<=len;k<<=1)

	{

		wn=cp(cos(2*pi*flg/k),sin(2*pi*flg/k));

		t=k>>1;

		for(i=0;i<len;i+=k)

		{

			w=cp(1,0);

			for(j=i;j<i+t;j++)

			{

				tmp=a[j+t]*w;

				a[j+t]=a[j]-tmp;

				a[j]=a[j]+tmp;

				w=w*wn;

			}

		}

	}

	if(flg==-1) for(i=0;i<len;i++) a[i].x/=len;

}

int main()

{

    int T,i,x;scanf("%d",&T);int len;

    while(T--)

    {

		memset(num,0,sizeof num);

		ll ans=0;

		maxx=0;

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

        n=rd(); for(i=1;i<=n;i++) x=rd(),b[i]=x,a[x].x++,maxx=max(x,maxx);

        len=1; while(len<(maxx<<1)) len<<=1;

        fft(a,len,1);

		for(i=0;i<len;i++) a[i]=a[i]*a[i];

		fft(a,len,-1);

        for(i=1;i<=n;i++) a[b[i]<<1].x--;

        for(i=1;i<=maxx;i++) num[i]=num[i-1]+(ll)(a[i].x/2+0.1);

        for(i=1;i<=n;i++) ans+=num[b[i]];

        printf("%.7f\n",1-(1.0*ans/(1.0*n*(n-1)/6*(n-2))));

    }

	return 0;

}