#include <bits/stdc++.h>
using namespace std;
const int N = 1e5 + 5;
int T, n, l, a[N];
int main() {
  scanf("%d", &T);
  while (T--) {
    scanf("%d%d", &n, &l);
    for (int i = 1; i <= n; i++) scanf("%d", &a[i]);
    int pl = 1, pr = n;
    double s = 0, t = l;
    double v1 = 1, v2 = 1;
    double ans = 0;
    while (pl <= pr) {
      if ((a[pl] - s) / v1 < (t - a[pr]) / v2) {
        ans += (a[pl] - s) / v1;
        t -= (a[pl] - s) / v1 * v2;
        s = a[pl];
        v1++;
        pl++;
      } else if ((a[pl] - s) / v1 > (t - a[pr]) / v2) {
        ans += (t - a[pr]) / v2;
        s += (t - a[pr]) / v2 * v1;
        t = a[pr];
        v2++;
        pr--;
      } else {
        ans += (a[pl] - s) / v1;
        s = a[pl], t = a[pr];
        v1++, v2++;
        pl++, pr--;
      }
    }
    ans += (t - s) / (v1 + v2);
    printf("%lf\n", ans);
  }
  return 0;
}
