#include <bits/stdc++.h>
using namespace std;
void solve() {
  int n, l;
  cin >> n >> l;
  vector<int> flags(n + 2);
  flags[0] = 0, flags[n + 1] = l;
  for (int i = 0; i < n; ++i) {
    cin >> flags[i + 1];
  }
  vector<double> s(n + 2);
  vector<double> e(n + 2);
  s[0] = 0;
  e[n + 1] = 0;
  for (int i = 1; i < n + 2; ++i) {
    s[i] = s[i - 1] + (flags[i] - flags[i - 1]) * 1.0 / i;
    e[n - i + 1] =
        e[n - i + 2] + (flags[n - i + 2] - flags[n - i + 1]) * 1.0 / (i);
  }
  for (int i = 0; i < n + 2; ++i) {
    if (abs(s[i] - e[i]) < 1e-6) {
      cout << s[i] << "\n";
      return;
    } else if (s[i] > e[i]) {
      double sspd = i;
      double espd = n - i + 2;
      double dist = flags[i] - flags[i - 1];
      double result = espd * e[i] + sspd * s[i - 1] + dist;
      result = result / (sspd + espd);
      cout << result << "\n";
      return;
    }
  }
}
int main() {
  cout << setprecision(9);
  cout << fixed;
  int tests;
  cin >> tests;
  while (tests) {
    solve();
    tests--;
  }
}
