#include <bits/stdc++.h>
using namespace std;
int main() {
  int t;
  cin >> t;
  while (t--) {
    int n, L;
    cin >> n >> L;
    double l = 0, r = L;
    vector<int> a(n);
    for (int i = 0; i < n; i++) {
      cin >> a[i];
    }
    for (int precision = 0; precision < 100; precision++) {
      double mid = 0.5 * (l + r);
      double tot = 0;
      double time = mid, x = 0, v = 1;
      for (int i = 0; i < n; i++) {
        double until = (a[i] - x) / v;
        if (until <= time) {
          time -= until;
          x = a[i];
          ++v;
        } else {
          break;
        }
      }
      x += time * v;
      tot += x;
      time = mid;
      x = L;
      v = 1;
      for (int i = n - 1; i >= 0; i--) {
        double until = (x - a[i]) / v;
        if (until <= time) {
          time -= until;
          x = a[i];
          ++v;
        } else {
          break;
        }
      }
      x -= time * v;
      tot += L - x;
      if (tot >= L) {
        r = mid;
      } else {
        l = mid;
      }
    }
    cout << fixed << setprecision(12) << 0.5 * (l + r) << '\n';
  }
}
