1
5
0 1 6
0 2 3
0 3 7
3 4 2

8.6

#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;
int const MAX = 1e4 + 5;
int n, cnt, head[MAX], num[MAX];
double sum;

struct EDGE
{
	int v, w, next;
}e[2 * MAX];

void Add(int u, int v, int w)
{
	e[cnt].v = v;
	e[cnt].w = w;
	e[cnt].next = head[u];
	head[u] = cnt ++;
}

void DFS(int u, int fa)
{
	num[u] = 1;
	for(int i = head[u]; i != -1; i = e[i].next)
	{	
		int v = e[i].v;
		int w = e[i].w;
		if(v != fa)
		{
			DFS(v, u);
			num[u] += num[v];
			sum += 1.0 * num[v] * (n - num[v]) * w;
		}
	}
}

int main()
{
	int T;
	scanf("%d", &T);
	while(T --)
	{
		cnt = 0;
		sum = 0;
		memset(head, -1, sizeof(head));
		scanf("%d", &n);
		for(int i = 0; i < n - 1; i++)
		{
			int u, v, w;
			scanf("%d %d %d", &u, &v, &w);
			Add(u, v, w);
			Add(v, u, w);
		}
		DFS(0, -1);
		printf("%.7f\n", sum / (1.0 * n * (n - 1) / 2.0));
	}
}