#include <bits/stdc++.h>
using namespace std;
vector<long long int> sieve;
vector<long long int> *Sieve(int N) {
  const long long int maxn = N;
  sieve.resize(maxn);
  for (long long int(i) = 0; (i) < (maxn); ++(i)) sieve[i] = 1;
  sieve[1] = 0;
  sieve[0] = 0;
  for (long long int(i) = 2; (i) <= (maxn); (i) += (1))
    if (1 == sieve[i])
      for (long long int j = i * i; j < maxn; j += i) sieve[j] = 0;
  vector<long long int> *primes = new vector<long long int>();
  primes->push_back(2);
  for (int i = 3; i < maxn; i += 2)
    if (sieve[i]) {
      primes->push_back(i);
    }
  return primes;
}
long long int extended_GCD(long long int a, long long int b, long long int &x,
                           long long int &y) {
  if (a == 0) {
    x = 0;
    y = 1;
    return b;
  }
  long long int x1, y1;
  long long int gcd = extended_GCD(b % a, a, x1, y1);
  x = y1 - (b / a) * x1;
  y = x1;
  return gcd;
}
long long int ncr(long long int n, long long int k) {
  long long int r = 1;
  if (k > n - k) k = n - k;
  for (long long int i = 0; i < k; i++) {
    r *= (n - i);
    r /= (i + 1);
  }
  return r;
}
long long int power(long long int a, long long int b,
                    long long int m = 1000000007) {
  long long int res = 1;
  while (b > 0) {
    if (b & 1) res = (res * a) % m;
    a = (a * a) % m;
    b >>= 1;
  }
  return res;
}
long long int modinv(long long int a, long long int mod = 1000000007) {
  long long int x, y;
  extended_GCD(a, mod, x, y);
  if (x < 0) x += mod;
  return x;
}
bool prime(int n) {
  int c = 0;
  for (int i = 2; (i * i) <= n; i++) {
    if (n % i == 0) {
      c++;
      return false;
    }
  }
  if (c == 0) return true;
}
vector<int> v[200006];
int vis[200006];
int c;
void dfs(int n) {
  vis[n] = 1;
  c++;
  for (int i : v[n]) {
    if (!vis[i]) dfs(i);
  }
}
int a[100001];
void solve() {
  int n, k;
  cin >> n >> k;
  int a[n];
  for (long long int(i) = 0; (i) < (n); ++(i)) cin >> a[i];
  int sa = 1, sb = 1;
  double ca = 0, cb = k;
  int i = 0, j = n - 1;
  double ans = 0;
  while (i <= j) {
    double t1, t2, d1, d2;
    d1 = a[i] - ca;
    d2 = cb - a[j];
    t1 = (double)d1 / sa;
    t2 = (double)d2 / sb;
    if (t1 <= t2) {
      ans += t1;
      ca += d1;
      i++;
      sa++;
      cb -= (double)t1 * sb;
    } else {
      ans += t2;
      cb -= d2;
      j--;
      sb++;
      ca += (double)t2 * sa;
    }
  }
  ans += double(cb - ca) / (sa + sb);
  cout << fixed << setprecision(10) << ans;
  cout << "\n";
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long int t;
  cin >> t;
  while (t--) solve();
  return 0;
}
