#include <bits/stdc++.h>
namespace chtholly {
char buf[1 << 23], *p1 = buf, *p2 = buf;
inline int read() {
  int x = 0, f = 1;
  char c = getchar();
  for (; !isdigit(c); c = getchar()) f ^= c == '-';
  for (; isdigit(c); c = getchar()) x = (x << 3) + (x << 1) + (c ^ '0');
  return f ? x : -x;
}
template <typename mitsuha>
inline bool read(mitsuha &x) {
  x = 0;
  int f = 1;
  char c = getchar();
  for (; !isdigit(c) && ~c; c = getchar()) f ^= c == '-';
  if (!~c) return 0;
  for (; isdigit(c); c = getchar()) x = (x << 3) + (x << 1) + (c ^ '0');
  return x = f ? x : -x, 1;
}
template <typename mitsuha>
inline int write(mitsuha x) {
  if (!x) return 0 & putchar(48);
  if (x < 0) putchar('-'), x = -x;
  int bit[20], i, p = 0;
  for (; x; x /= 10) bit[++p] = x % 10;
  for (i = p; i; --i) putchar(bit[i] + 48);
  return 0;
}
inline char fuhao() {
  char c = getchar();
  for (; isspace(c); c = getchar())
    ;
  return c;
}
}  // namespace chtholly
using namespace chtholly;
using namespace std;
const int yuzu = 1e5;
typedef int fuko[yuzu | 10];
fuko p;
int judge(double x, int n, int len) {
  int i, j, v;
  double t1 = x, d1 = 0, d2 = len;
  for (v = i = 1; i <= n + 1 && t1 > 0; ++i, ++v) {
    if (t1 >= 1.0 * (p[i] - p[i - 1]) / v) {
      d1 = p[i];
      t1 -= 1.0 * (p[i] - p[i - 1]) / v;
    } else {
      d1 += t1 * v;
      t1 = 0;
    }
  }
  t1 = x;
  for (v = 1, i = n; ~i && t1 > 0; --i, ++v) {
    if (t1 >= 1.0 * (p[i + 1] - p[i]) / v) {
      d2 = p[i];
      t1 -= 1.0 * (p[i + 1] - p[i]) / v;
    } else {
      d2 -= t1 * v;
      t1 = 0;
    }
  }
  return d1 >= d2;
}
int main() {
  for (int t = read(); t--;) {
    int i, n = read(), len = read();
    for (i = 1; i <= n; ++i) p[i] = read();
    p[n + 1] = len;
    double l = 0, r = 1e9, mid;
    for (i = 1; i <= 70; ++i) {
      mid = (l + r) / 2;
      judge(mid, n, len) ? r = mid : l = mid;
    }
    printf("%.15lf\n", l);
  }
}
