#include <bits/stdc++.h>
using namespace std;
int t;
int n, l;
int a[100005];
int main() {
  scanf("%d", &t);
  while (t--) {
    scanf("%d%d", &n, &l);
    a[0] = 0;
    for (int i = 1; i <= n; i++) scanf("%d", &a[i]);
    a[n + 1] = l;
    double lb = 0, rb = l;
    for (int i = 1; i <= 50; i++) {
      double mid = (lb + rb) / 2;
      double ldist = 0, rdist = l;
      double tme = mid;
      for (int j = 1; j <= n + 1; j++) {
        if ((a[j] - ldist) / (double)j <= tme) {
          tme -= (a[j] - ldist) / (double)j;
          ldist = a[j];
        } else {
          ldist += j * tme;
          tme = 0;
          break;
        }
      }
      tme = mid;
      for (int j = n; j >= 0; j--) {
        if ((rdist - a[j]) / (double)(n - j + 1) <= tme) {
          tme -= (rdist - a[j]) / (double)(n - j + 1);
          rdist = a[j];
        } else {
          rdist -= (n - j + 1) * tme;
          tme = 0;
          break;
        }
      }
      if (ldist <= rdist) {
        lb = mid;
      } else {
        rb = mid;
      }
    }
    printf("%.7f\n", lb);
  }
  return 0;
}
