#include <bits/stdc++.h>
using namespace std;
void solve() {
  int n, l;
  cin >> n >> l;
  int a[n];
  for (int i = 0; i < n; i++) cin >> a[i];
  int i = 0, j = n - 1;
  long double t = 0, s1 = 0, s2 = l, v2 = 1, v1 = 1;
  while (i <= j) {
    long double t1 = (a[i] - s1) / v1;
    long double t2 = (s2 - a[j]) / v2;
    long double p = min(t1, t2);
    s1 += (v1 * p);
    s2 -= (v2 * p);
    t += min(t1, t2);
    if (t1 <= t2) {
      i++;
      v1++;
    }
    if (t1 >= t2) {
      j--;
      v2++;
    }
  }
  t += (s2 - s1) / (v2 + v1);
  cout << t << endl;
}
int main() {
  int tc;
  scanf("%d", &tc);
  while (tc--) {
    cout << fixed << setprecision(17);
    solve();
  }
  return 0;
}
