#include <bits/stdc++.h>
using namespace std;
template <typename T, typename S>
ostream &operator<<(ostream &os, const pair<T, S> &p) {
  return os << "(" << p.first << ", " << p.second << ")";
}
template <typename C, typename T = decay<decltype(*begin(declval<C>()))>,
          typename enable_if<!is_same<C, string>::value>::type * = nullptr>
ostream &operator<<(ostream &os, const C &c) {
  bool f = true;
  os << "[";
  for (const auto &x : c) {
    if (!f) os << ", ";
    f = false;
    os << x;
  }
  return os << "]";
}
template <typename T>
void debug(string s, T x) {
  cerr << s << " = " << x << "\n";
}
template <typename T, typename... Args>
void debug(string s, T x, Args... args) {
  cerr << s.substr(0, s.find(',')) << " = " << x << " | ";
  debug(s.substr(s.find(',') + 2), args...);
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  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];
    long double l = 0, r = 1e9;
    for (int rep = 0; rep < 200; rep++) {
      long double m = (l + r) / 2, lpos = 0, ti = m;
      for (int i = 0; i < n; i++) {
        long double cur =
            min(ti, (a[i] - (i > 0 ? a[i - 1] : 0)) / (long double)(i + 1));
        lpos += cur * (i + 1);
        ti -= cur;
      }
      long double fin = min(ti, (L - a[n - 1]) / (long double)(n + 1));
      lpos += fin * (n + 1);
      long double rpos = L;
      ti = m;
      for (int i = n - 1; i >= 0; i--) {
        long double cur =
            min(ti, ((i < n - 1 ? a[i + 1] : L) - a[i]) / (long double)(n - i));
        rpos -= cur * (n - i);
        ti -= cur;
      }
      fin = min(ti, a[0] / (long double)(n + 1));
      rpos -= fin * (n + 1);
      if (lpos < rpos)
        l = m;
      else
        r = m;
    }
    cout << fixed << setprecision(10) << l << "\n";
  }
  return 0;
}
