#include <bits/stdc++.h>
using namespace std;
int main() {
  int tt;
  cin >> tt;
  while (tt--) {
    int n, l;
    cin >> n >> l;
    vector<double> flags(n);
    for (auto& i : flags) cin >> i;
    double a_s = 0, b_s = l;
    double a_v = 1, b_v = 1;
    int a_i = 0, b_i = n - 1;
    double t = 0;
    for (;;) {
      double a_d = a_i < n ? flags[a_i] - a_s : 1e300,
             b_d = b_i >= 0 ? b_s - flags[b_i] : 1e300;
      double a_t = a_d / a_v, b_t = b_d / b_v;
      double sep = b_s - a_s;
      double sep_t = sep / (a_v + b_v);
      double mn = min({a_t, b_t, sep_t});
      t += mn;
      a_s += a_v * mn;
      b_s -= b_v * mn;
      if (a_t == mn) {
        a_v++;
        a_i++;
      } else if (b_t == mn) {
        b_v++;
        b_i--;
      } else
        break;
    }
    cout << fixed << t << endl;
  }
}
