#include <bits/stdc++.h>
using namespace std;
long long int n, l;
bool compare_float(long double x, long double y,
                   long double epsilon = 0.0000001f) {
  if (fabs(x - y) < epsilon) return true;
  return false;
}
void advmod(long long int &a, long long int b) { a = (a % b + b) % b; }
bool isPrime(long long int n) {
  if (n == 1) return false;
  if (n == 2) return true;
  for (long long int i = 2; i * i <= n; i++) {
    if (n % i == 0) return false;
  }
  return true;
}
long long int power(long long int x, long long int y, long long int p) {
  long long int res = 1;
  x = x % p;
  if (x == 0) return 0;
  while (y > 0) {
    if (y & 1) res = (res * x) % p;
    y = y >> 1;
    x = (x * x) % p;
  }
  return res;
}
long double car0(long double d, long double *a) {
  long double ret = 0, s = 1, b = 1;
  for (long long int i = 1; i <= n; i++) {
    if (a[i] < d) {
      ret += ((a[i] - a[i - 1]) / s);
    } else {
      b = 0;
      ret += ((d - a[i - 1]) / s);
      break;
    }
    s++;
  }
  if (b) ret += ((d - a[n]) / s);
  return ret;
}
long double carl(long double d, long double *a) {
  long double cur = l, dist = 0, s = 1, ret = 0;
  for (long long int i = n; i >= 0; i--) {
    if (dist + cur - a[i] < d) {
      ret += ((cur - a[i]) / s);
    } else {
      ret += ((d - dist) / s);
      break;
    }
    dist += (cur - a[i]);
    cur = a[i];
    s++;
  }
  return ret;
}
long double ans(long double hi, long double lo, long double *a) {
  while (hi > lo) {
    long double mid = (lo + hi) / 2;
    if (compare_float(car0(mid, a), carl(l - mid, a)))
      return mid;
    else if (car0(mid, a) < carl(l - mid, a))
      lo = mid;
    else
      hi = mid;
  }
}
void solve() {
  cout << fixed << setprecision(15);
  cin >> n >> l;
  long double t[n + 1], a[n + 1];
  for (long long int i = 1; i <= n; i++) cin >> a[i];
  a[0] = 0;
  long double fd = ans(l, 0, a);
  cout << car0(fd, a) << '\n';
}
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long int t = 1;
  cin >> t;
  while (t--) {
    solve();
  }
  return 0;
}
