#include <bits/stdc++.h>
using namespace std;
void solve() {
  long double n, l;
  cin >> n >> l;
  long double a[100005];
  for (int i = 1; i <= n; ++i) cin >> a[i];
  a[0] = 0;
  a[(int)n + 1] = l;
  long double low = 0;
  long double high = l;
  while (low <= high) {
    long double mid = (low + high) / 2;
    int st = 0;
    int ed = n + 1;
    long double ls = 1, rs = 1;
    long double lt = 0, rt = 0;
    while (st + 1 <= n and a[st + 1] <= mid) {
      lt += (a[st + 1] - a[st]) / ls;
      ls++;
      st++;
    }
    while (ed - 1 >= 1 and a[ed - 1] >= mid) {
      rt += (a[ed] - a[ed - 1]) / rs;
      rs++;
      ed--;
    }
    lt += (mid - a[st]) / ls;
    rt += (a[ed] - mid) / rs;
    if (fabs(rt - lt) <= 0.000001) {
      cout << fixed << setprecision(10) << lt << "\n";
      break;
    }
    if (lt > rt)
      high = mid - 0.0000001;
    else {
      low = mid + 0.0000001;
    }
  }
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int t = 1;
  cin >> t;
  while (t--) solve();
  return 0;
}
