#include<cstdio>

#include<cstring>

#include<algorithm>

#include<cmath>

#define db double

#define ll long long

using namespace std;

const int N=1e5+,M=(<<)+;

const db pi=acos(-);

int T,n,len,r[M],f[N];

struct cpl{db x,y;}a[M],I;

cpl operator+ (cpl a,cpl b){return (cpl){a.x+b.x,a.y+b.y};}

cpl operator- (cpl a,cpl b){return (cpl){a.x-b.x,a.y-b.y};}

cpl operator* (cpl a,cpl b){return (cpl){a.x*b.x-a.y*b.y,a.x*b.y+a.y*b.x};}

int Mx(int a,int b){return a>b?a:b;}

int rdn()

{

  int ret=;bool fx=;char ch=getchar();

  while(ch>''||ch<''){if(ch=='-')fx=;ch=getchar();}

  while(ch>=''&&ch<='')ret=ret*+ch-'',ch=getchar();

  return fx?ret:-ret;

}

void fft(cpl *a,bool fx)

{

  for(int i=;i<len;i++)

    if(i<r[i])swap(a[i],a[r[i]]);

  for(int R=;R<=len;R<<=)

    {

      int m=R>>;

      cpl wn=(cpl){ cos(pi/m),fx?-sin(pi/m):sin(pi/m) };

      for(int i=;i<len;i+=R)

    {

      cpl w=I;

      for(int j=;j<m;j++,w=w*wn)

        {

          cpl x=a[i+j],y=w*a[i+m+j];

          a[i+j]=x+y; a[i+m+j]=x-y;

        }

    }

    }

  if(!fx)return;

  for(int i=;i<len;i++)a[i].x/=len;

}

int main()

{

  T=rdn();int mx=; I.x=;

  while(T--)

    {

      n=rdn();

      for(int i=;i<len;i++)a[i].x=a[i].y=;

      for(int i=;i<=mx;i++)f[i]=;

      for(int i=,u;i<=n;i++)

    {

      u=rdn();mx=Mx(mx,u);

      a[u].x+=; f[u]++;

    }

      for(len=;len<=mx<<;len<<=);//mx

      for(int i=,j=len>>;i<len;i++)r[i]=(r[i>>]>>)+((i&)?j:);

      fft(a,);

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

      fft(a,);

      ll sm=(ll)n*(n-)*(n-),ans=sm,lj=;

      for(int i=;i<=mx;i++)

    {

      lj+=( (ll)(a[i].x+0.5)-((i&)==?f[i>>]:) )*;

      ans-=lj*f[i];//*f[i]!!

    }

      printf("%.7f\n",(db)ans/sm);

    }

  return ;

}