#include <bits/stdc++.h>
using namespace std;
const int mod = 20090717;
const int maxn = 1e5 + 50;
int n;
double l;
double a[maxn];
double rt[maxn];
int main() {
  int T;
  cin >> T;
  while (T--) {
    cin >> n >> l;
    a[n + 1] = l;
    rt[n + 1] = 0;
    for (int i = 1; i <= n; i++) {
      cin >> a[i];
    }
    int x = 1;
    for (int i = n; i >= 0; i--) {
      rt[i] = rt[i + 1] + 1.0 * (a[i + 1] - a[i]) / x;
      ++x;
    }
    double lt = 0;
    x = 1;
    int t = 0;
    for (int i = 1; i <= n + 1; i++) {
      lt += 1.0 * (a[i] - a[i - 1]) / i;
      if (lt > rt[i]) {
        t = i - 1;
        lt -= 1.0 * (a[i] - a[i - 1]) / i;
        break;
      }
    }
    double ans = 1.0 *
                 (1.0 * (t + 1) * lt + 1.0 * (n - t + 1) * rt[t + 1] +
                  1.0 * (a[t + 1] - a[t])) /
                 (n + 2);
    printf("%.10lf\n", ans);
  }
  return 0;
}
