#include <bits/stdc++.h>
using namespace std;
void solve() {
  long long int n, i, m, q, mx = INT_MIN, mn = INT_MAX, k, j, sm = 0, pdt = 1;
  long double t = 0;
  cin >> n;
  long long int v[n];
  cin >> m;
  for (i = 0; i < n; i++) {
    cin >> v[i];
  }
  i = 0;
  j = n - 1;
  long double vi = 1, vj = 1, c = 0, d = 0;
  long double li = 0, lj = m;
  while (true) {
    if ((i - j) == 2) {
      cout << fixed << setprecision(9) << t << "\n";
      return;
    }
    if ((i - j) == 1) {
      t += (lj - li) / (vi + vj);
      cout << fixed << setprecision(9) << t << "\n";
      return;
    }
    c = ((long double)v[i] - li) / vi;
    d = (lj - (long double)v[j]) / vj;
    if (c == d) {
      t += c;
      li = v[i];
      lj = v[j];
      i++;
      j--;
      vi++;
      vj++;
    } else if (c < d) {
      t += c;
      lj -= c * vj;
      li = v[i];
      vi++;
      i++;
    } else {
      t += d;
      li += d * vi;
      lj = v[j];
      vj++;
      j--;
    }
  }
  return;
}
int main() {
  int t;
  cin >> t;
  while (t--) {
    solve();
  }
  return 0;
}
