#include <bits/stdc++.h>
using namespace std;
const int N = 2e5 + 10;
const int inf = 0x3f3f3f3f;
const int mod = 998244353;
int a[N], n, L;
bool check(double x, int id) {
  double s1 = 0, s2 = 0;
  double t = x;
  int s = 1;
  for (int i = 1; i <= n + 1; i++) {
    double cur = 1.0 * (a[i] - a[i - 1]) / s;
    if (cur <= t) {
      s1 = a[i];
      t -= cur;
    } else {
      s1 += t * s;
      break;
    }
    s++;
  }
  s = 1;
  t = x;
  for (int i = n; i >= 0; i--) {
    double cur = 1.0 * (a[i + 1] - a[i]) / s;
    if (cur <= t) {
      s2 = L - a[i];
      t -= cur;
    } else {
      s2 += t * s;
      break;
    }
    s++;
  }
  return s1 + s2 >= 1.0 * L;
}
void solve() {
  scanf("%d%d", &n, &L);
  for (int i = 1; i <= n; i++) {
    scanf("%d", &a[i]);
  }
  a[n + 1] = L;
  double l = 0, r = L;
  double res = 1e9;
  for (int i = 1; i <= 100; i++) {
    double mid = 1.0 * (l + r) / 2;
    if (check(mid, i)) {
      res = mid;
      r = mid;
    } else
      l = mid;
  }
  printf("%.6f\n", res);
}
int main() {
  int t;
  cin >> t;
  while (t--) solve();
  return 0;
}
