#include <bits/stdc++.h>
using namespace std;
long long int gcd(long long int a, long long int b) {
  return (b == 0LL ? a : gcd(b, a % b));
}
long double dist(long double x, long double arayikhalatyan, long double x2,
                 long double y2) {
  return sqrt((x - x2) * (x - x2) +
              (arayikhalatyan - y2) * (arayikhalatyan - y2));
}
long long int S(long long int a) { return (a * (a + 1LL)) / 2; }
mt19937 rnd(chrono::steady_clock::now().time_since_epoch().count());
char vow[] = {'a', 'e', 'i', 'o', 'u'};
int dx[] = {0, -1, 0, 1, -1, -1, 1, 1, 0};
int dy[] = {-1, 0, 1, 0, -1, 1, -1, 1, 0};
const int N = 1e5 + 20;
const long long int mod = 1e9 + 7;
const long double pi = acos(-1);
const int T = 238;
const long double e = 1e-13;
long long int bp(long long int a, long long int b = mod - 2LL) {
  long long int ret = 1;
  while (b) {
    if (b & 1) ret *= a, ret %= mod;
    a *= a;
    a %= mod;
    b >>= 1;
  }
  return ret;
}
ostream& operator<<(ostream& c, pair<int, int> a) {
  c << a.first << " " << a.second;
  return c;
}
struct pt {
  long long int x, y;
  int ind;
  int bl;
  bool operator==(const pt& a) { return ind == a.ind; }
  bool operator<(const pt& b) {
    if (x == b.x) return y < b.y;
    return x < b.x;
  }
};
int t, n, k;
long double a[N], dp[N], dp1[N];
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  ;
  setprecision(9);
  cout.setf(ios::showpoint);
  cout.setf(ios::fixed);
  cin >> t;
  while (t--) {
    cin >> n >> k;
    for (int i = 0; i < n; i++) {
      cin >> a[i];
      if (i == 0)
        dp[i] = a[i];
      else
        dp[i] = dp[i - 1] + (a[i] - a[i - 1]) / (1.0 + (long double)(i));
    }
    dp[n] = dp[n - 1] + (k - a[n - 1]) / (1.0 + (long double)(n));
    dp1[n] = 0;
    a[n] = k;
    dp1[n - 1] = k - a[n - 1];
    for (int i = n - 2; i >= 0; i--) {
      dp1[i] = dp1[i + 1] + (a[i + 1] - a[i]) / (long double)(n - i);
    }
    for (int i = 0; i <= n; i++) {
      if (dp[i] == dp1[i]) {
        cout << dp[i];
        break;
      }
      if (dp[i] > dp1[i]) {
        long double l = 0;
        if (i) l = a[i - 1];
        long double r = a[i];
        long double t1 = 0;
        if (i) t1 = dp[i - 1];
        long double t2 = dp1[i];
        l += max((long double)0, t2 - t1) * (long double)(1.0 + i);
        r -= max((long double)0, t1 - t2) * (long double)(n - i + 1.0);
        cout << max(t1, t2) + (r - l) / (long double)(n + 2.0);
        break;
      }
    }
    cout << endl;
  }
  return 0;
}
