#include<cstdio>
#include<cmath>
#include<iostream>
#include<cstring>
#include<algorithm>
#include<string>
#include<stack>
#include<queue>
#include<map>
#include<utility>
#pragma comment(linker, "/STACK:102400000,102400000")
using namespace std;
const int MN=300+10;
const int ME=100000+10;
const int MOD=2520;
typedef long long LL;
typedef unsigned long long ULL;
const double pi=acos(-1.0);
const int Intmax=(~(1<<31));
const int Intmin=(1<<31);
const LL LLmax=(~(1LL<<63));
const LL LLmin=(1LL<<63);
const ULL ULLmax=(~(0ULL));
typedef long long LL;
int a[MN];
int main(){
#ifndef ONLINE_JUDGE
    freopen("1.cpp","r",stdin);
#endif // ONLINE_JUDGE
    int t;
    scanf("%d",&t);
    while(t--){
        int n;
        scanf("%d",&n);
        memset(a,0,sizeof(a));
        int m=n;
        while(m--){
            int x;
            scanf("%d",&x);
            a[x]++;
        }
        int sum=0;
        int ans=0,i;
        for(i=300;i>=1;i--){
            if(a[i]){
                sum+=a[i];
                ans+=(n-sum)*a[i];
            }
        }
        double res=ans*1.0/(n*(n-1));
        printf("%.6f\n",res);
    }
    return 0;
}