#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  int t;
  cin >> t;
  while (t--) {
    int n, l;
    cin >> n >> l;
    double a[n + 2];
    a[0] = 0;
    a[n + 1] = l;
    for (int i = 1; i <= n; i++) {
      cin >> a[i];
    }
    double timel[n + 2], timer[n + 2];
    int start = 0;
    double speed = 1;
    timel[0] = timer[n + 1] = 0;
    for (int i = 0; i <= n; i++) {
      timel[i + 1] = timel[i] + double(a[i + 1] - a[i]) / speed;
      speed += 1;
    }
    speed = 1;
    for (int i = n + 1; i > 0; i--) {
      timer[i - 1] = timer[i] + double(a[i] - a[i - 1]) / speed;
      speed += 1;
    }
    cout << fixed;
    cout.precision(10);
    for (int i = 0; i < n + 1; i++) {
      if (timel[i] <= timer[i] and timer[i + 1] <= timel[i + 1]) {
        double ll = 0, rr = l + 1;
        for (int j = 0; j < 60; j++) {
          double m = (ll + rr) / 2;
          double left = a[i] + (i + 1) * m;
          double right = a[i + 1] - (timel[i] + m - timer[i + 1]) * (n + 1 - i);
          if (left < right)
            ll = m;
          else
            rr = m;
        }
        cout << timel[i] + ll << '\n';
        break;
      }
    }
  }
  return 0;
}
