#include <bits/stdc++.h>
using namespace std;
const long long MOD = pow(10, 9) + 7;
const long long mod1 = 998244353;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int tt;
  cin >> tt;
  while (tt--) {
    long long n, l, x = 0, a;
    cin >> n >> l;
    vector<long long> v1, v2;
    long double t1[n + 2], t2[n + 2];
    t1[0] = 0;
    t2[n + 1] = 0;
    for (long long i = 1; i <= n; i++) {
      cin >> a;
      v1.push_back(a - x);
      v2.push_back(a - x);
      x = a;
    }
    v1.push_back(l - a);
    v2.push_back(l - a);
    reverse(v2.begin(), v2.end());
    for (long long i = 0; i < n + 1; i++) {
      long double e = i;
      t1[i + 1] = t1[i] + v1[i] / (e + 1);
      t2[n - i] = t2[n - i + 1] + v2[i] / (e + 1);
    }
    long long o = n;
    ;
    for (long long i = 0; i <= n; i++) {
      if (t1[i] <= t2[i] && t1[i + 1] > t2[i + 1]) {
        o = i;
        break;
      }
    }
    long double dis = v1[o] + (o + 1) * (t1[o] - t2[o + 1]);
    long double qt = (v1[o] + (o + 1) * (t1[o] - t2[o + 1])) / (n + 2);
    long double ans =
        (v1[o] + (o + 1) * (t1[o] - t2[o + 1])) / (n + 2) + t2[o + 1];
    std::cout << std::setprecision(10) << std::fixed
              << (v1[o] + (o + 1) * (t1[o] - t2[o + 1])) / (n + 2) + t2[o + 1];
    cout << endl;
  }
}
