#include <bits/stdc++.h>
using namespace std;
template <class c>
struct rge {
  c b, e;
};
template <class c>
rge<c> range(c i, c j) {
  return rge<c>{i, j};
}
template <class c>
auto dud(c *x) -> decltype(cerr << *x, 0);
template <class c>
char dud(...);
struct debug {
  template <class c>
  debug &operator<<(const c &) {
    return *this;
  }
};
void solve() {
  long long n, l;
  cin >> n >> l;
  vector<long double> a(n);
  for (int i = 0; i < n; ++i) {
    cin >> a[i];
  }
  long double left = 0, rigth = 2e9, mid;
  while (abs(rigth - left) >= 1e-7) {
    mid = (left + rigth) / 2.0;
    long double s = l;
    long double m = mid;
    long double f = 0, cnt = 1;
    for (int i = 0; i < n; ++i) {
      long double p = (a[i] - f) / (cnt);
      if (p <= m) {
        m -= p;
        f = a[i];
        cnt++;
      } else {
        break;
      }
    }
    f += cnt * m;
    m = mid;
    cnt = 1;
    for (int i = n - 1; i >= 0; --i) {
      long double p = (s - a[i]) / (cnt);
      if (p <= m) {
        m -= p;
        cnt++;
        s = a[i];
      } else {
        break;
      }
    }
    s -= (cnt * m);
    if (s <= f) {
      rigth = mid;
    } else
      left = mid;
  }
  cout << fixed << setprecision(15) << rigth << '\n';
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int t;
  cin >> t;
  while (t--) solve();
}
