#include <bits/stdc++.h>
using namespace std;
double fact[1000];
long long n, ans[160][160][160];
long long ar[1000];
long long s, ms;
double answ;
int main() {
  ios_base::sync_with_stdio(0);
  cin >> n;
  fact[0] = 1;
  for (int i = 1; i <= 55; i++) fact[i] = fact[i - 1] * i;
  for (int i = 1; i <= n; i++) {
    cin >> ar[i];
    s += ar[i];
  }
  cin >> ms;
  if (s <= ms)
    cout << n << endl;
  else {
    for (int iter = 1; iter <= n; iter++) {
      for (int i = 0; i <= 50; i++)
        for (int j = 0; j <= 50; j++)
          for (int q = 0; q <= 60; q++) ans[i][j][q] = 0;
      ans[0][0][0] = 1;
      for (int i = 1; i <= n; i++)
        for (int j = 0; j <= i; j++)
          for (int t = 0; t <= 50; t++)
            if (ans[i - 1][j][t]) {
              ans[i][j][t] += ans[i - 1][j][t];
              if (i != iter) ans[i][j + 1][t + ar[i]] += ans[i - 1][j][t];
            }
      for (int j = 0; j <= n; j++)
        for (int t = 0; t <= ms; t++)
          if (t + ar[iter] > ms) {
            answ += ans[n][j][t] / fact[n] * fact[j] * fact[n - j - 1] * j;
          }
    }
    cout.precision(6);
    cout << fixed << answ << endl;
  }
  cin.get();
  cin.get();
  return 0;
}
