#include <bits/stdc++.h>
using namespace std;
const double EPS = 1e-6;
int T, n, l;
int ar[100003];
int main() {
  scanf("%d", &T);
  for (int tc = 1; tc <= T; tc++) {
    scanf("%d%d", &n, &l);
    for (int i = 1; i <= n; i++) scanf("%d", &ar[i]);
    double ans = 0;
    double x1 = 0, x2 = l;
    double s1 = 1, s2 = 1;
    for (int i = 1, j = n; i <= j;) {
      double t1 = (ar[i] - x1) / s1;
      double t2 = (x2 - ar[j]) / s2;
      if (fabs(t1 - t2) < EPS) {
        i++;
        j--;
        ans += t1;
        x1 += t1 * s1;
        x2 -= t1 * s2;
        s1 += 1;
        s2 += 1;
      } else if (t1 < t2) {
        i++;
        ans += t1;
        x1 += t1 * s1;
        x2 -= t1 * s2;
        s1 += 1;
      } else {
        j--;
        ans += t2;
        x1 += t2 * s1;
        x2 -= t2 * s2;
        s2 += 1;
      }
    }
    ans += (x2 - x1) / (s1 + s2);
    printf("%lf\n", ans);
  }
  return 0;
}
