#include <bits/stdc++.h>
using namespace std;
const double EXP = 1e-7;
long long T;
long long n, l, r;
double nowl, nowr, sl, sr;
double a[100001];
double t, tmp;
int main() {
  scanf("%lld", &T);
  while (T--) {
    t = 0.0;
    nowl = 0.0;
    l = 1;
    sl = sr = 1.0;
    cin >> n >> nowr;
    r = n;
    for (int i = 1; i <= n; ++i) cin >> a[i];
    while (1) {
      if (l > r || (l == r && (a[l] <= nowl || a[r] >= nowr))) {
        t += (nowr - nowl) / (sl + sr);
        break;
      }
      tmp = min((a[l] - nowl) / sl, (nowr - a[r]) / sr);
      t += tmp;
      nowl += tmp * sl;
      nowr -= tmp * sr;
      if (fabs(a[l] - nowl) <= EXP) l++, sl++;
      if (fabs(a[r] - nowr) <= EXP) r--, sr++;
    }
    printf("%.7lf\n", t);
  }
  return 0;
}
