#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int t;
  cin >> t;
  while (t--) {
    long long n, l;
    cin >> n >> l;
    long long a[n];
    for (int i = 0; i < n; i++) {
      cin >> a[i];
    }
    int i = 0, j = n - 1;
    double sp = 0, ep = l, ss = 1, es = 1, t = 0;
    while (i <= j) {
      double d1 = a[i] - sp;
      double d2 = ep - a[j];
      double t1 = d1 / ss;
      double t2 = d2 / es;
      if (t2 > t1) {
        ss++;
        sp = a[i];
        ep -= es * t1;
        i++;
        t += t1;
      } else {
        es++;
        ep = a[j];
        sp += ss * t2;
        j--;
        t += t2;
      }
    }
    t += (ep - sp) / (ss + es);
    cout << fixed << setprecision(10) << t << endl;
  }
  return 0;
}
