#include <bits/stdc++.h>
using namespace std;
void loop() {
  int n;
  long long int l;
  cin >> n >> l;
  long double a[n + 1];
  for (int i = 0; i < n; i++) cin >> a[i];
  long double time = 0;
  long double sp1 = 1, sp2 = 1, x1 = 0, x2 = l;
  int i = 0, j = n - 1;
  while (i <= j) {
    long double x = abs(a[i] - x1) / sp1;
    long double y = abs(a[j] - x2) / sp2;
    if (x == y) {
      time += x;
      x1 = a[i];
      x2 = a[j];
      i++;
      j--;
      sp1++;
      sp2++;
    } else if (x > y) {
      time += y;
      x2 = a[j];
      j--;
      sp2++;
      x1 += (y * sp1);
    } else {
      time += x;
      x1 = a[i];
      i++;
      sp1++;
      x2 -= (x * sp2);
    }
  }
  time += abs(x1 - x2) / (sp1 + sp2);
  cout << setprecision(12) << time << "\n";
}
int main() {
  int t;
  cin >> t;
  while (t--) loop();
}
