#include <bits/stdc++.h>
using namespace std;
void solve() {
  int n;
  double l;
  cin >> n >> l;
  vector<double> a(n + 2, 0);
  for (int i = 1; i <= n; i++) cin >> a[i];
  a[n + 1] = l;
  auto func = [&](int k) {
    double t1 = 0;
    for (int i = 1; i <= k; i++) {
      t1 += (a[i] - a[i - 1]) / i;
    }
    t1 *= (k + 1);
    double t2 = 0;
    for (int i = k + 1; i <= n; i++) {
      t2 += (a[i + 1] - a[i]) / (n + 1 - i);
    }
    t2 *= (n - k + 1);
    double ans = (t1 + t2 + a[k + 1] - a[k]) / (n + 2);
    return ans;
  };
  int lo = -1, hi = n;
  while (hi - lo > 1) {
    int mid = (lo + hi) / 2;
    if (mid < n && func(mid) <= func(mid + 1)) {
      hi = mid;
    } else {
      lo = mid;
    }
  }
  cout << fixed << setprecision(7) << func(hi);
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  ;
  int t;
  cin >> t;
  while (t--) {
    solve();
    cout << '\n';
  }
  return 0;
}
