#include <bits/stdc++.h>
using namespace std;
long double a[100005], ans, l;
int n;
const long double eps = 1e-8;
signed main() {
  register int i, T;
  scanf("%d", &T);
  while (T--) {
    scanf("%d%Lf", &n, &l);
    for (i = 1; i <= n; i++) scanf("%Lf", &a[i]);
    long double L = 0, R = l;
    ans = 0;
    int l = 0, r = n + 1, vl = 1, vr = 1;
    while (r - l > 1) {
      long double minl = a[l + 1] - L;
      long double minr = R - a[r - 1];
      if (minl / vl - minr / vr <= -eps)
        ans += minl / vl, l++, L = a[l], R -= minl / vl * vr, vl++;
      else if (minl / vl - minr / vr <= eps)
        ans += minl / vl, vl++, vr++, l++, r--, L = a[l], R = a[r];
      else
        ans += minr / vr, r--, R = a[r], L += minr / vr * vl, vr++;
    }
    ans += (R - L) / (vl + vr);
    printf("%.12Lf\n", ans);
  }
  return 0;
}
