#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  int t;
  cin >> t;
  while (t--) {
    int n, l;
    cin >> n >> l;
    vector<int> a(n);
    for (int i = 0; i < n; i++) {
      cin >> a[i];
    }
    int il = 0, ir = n - 1, sl = 1, sr = 1;
    double time = 0, pl = 0, pr = l;
    while (il <= ir) {
      double tl = (a[il] - pl) / sl, tr = (pr - a[ir]) / sr;
      if (tl < tr) {
        pl = a[il];
        pr -= sr * tl;
        il++;
        sl++;
        time += tl;
      } else {
        pr = a[ir];
        pl += sl * tr;
        ir--;
        sr++;
        time += tr;
      }
    }
    cout << fixed << setprecision(15) << time + (pr - pl) / (sr + sl) << '\n';
  }
  return 0;
}
