#include <bits/stdc++.h>
using namespace std;
struct keycompare {
  bool operator()(const pair<long long, long long>& v, const long long& k) {
    return (v.first < k);
  }
  bool operator()(const long long& k, const pair<long long, long long>& v) {
    return (k < v.first);
  }
};
long long mod1 = 998244353, mod2 = 1000000007, limit = 9223372036854775807;
long double pi = 3.1415926535897932;
long long modpow(long long x, long long n, long long m) {
  if (x > m) {
    x %= m;
  }
  if (n == 0) return 1 % m;
  long long u = modpow(x, n / 2, m);
  u = (u * u) % m;
  if (n % 2 == 1) u = (u * x) % m;
  return u;
}
long long gcd(long long a, long long b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
bool isprime(long long n) {
  if (n == 2) return true;
  for (long long i = 2; i * i <= n; i++) {
    if (n % i == 0) return false;
  }
  return true;
}
long long power(long long x, long long n) {
  long long x_n = 1;
  for (long long i = 0; i < n; i++) {
    x_n *= x;
  }
  return x_n;
}
long double arr[100005];
int n;
long double l;
long double time1(long double mid) {
  long double last = 1;
  long double timer = 0;
  for (int i = 1; i <= n + 1; i++) {
    if (arr[i] < mid) {
      timer += (arr[i] - arr[i - 1]) / last;
    } else {
      timer += (mid - arr[i - 1]) / last;
      break;
    }
    last++;
  }
  return timer;
}
long double time2(long double mid) {
  long double last = 1;
  long double timer = 0;
  for (int i = n; i >= 0; i--) {
    if (arr[i] > mid) {
      timer += (arr[i + 1] - arr[i]) / last;
    } else {
      timer += (arr[i + 1] - mid) / last;
      break;
    }
    last++;
  }
  return timer;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int t;
  cin >> t;
  while (t--) {
    cin >> n >> l;
    arr[0] = 0;
    arr[n + 1] = l;
    for (int i = 1; i <= n; i++) {
      cin >> arr[i];
    }
    long double left = 0, right = l;
    while (abs(left - right) > 1e-9) {
      long double mid = (left + right) / 2;
      if (time1(mid) < time2(mid)) {
        left = mid;
      } else {
        right = mid;
      }
    }
    cout << setprecision(15) << time1(left) << endl;
  }
}
