#include <bits/stdc++.h>
#pragma GCC optimization("O3")
#pragma GCC optimization("unroll-loops")
using namespace std;
template <class T>
using vec = std::vector<T>;
bool __hack = std::ios::sync_with_stdio(false);
auto __hack1 = cin.tie(nullptr);
struct Input {
  Input(istream &in) : in(&in) {}
  template <class T>
  T next() const {
    T x;
    *in >> x;
    return x;
  }
  int ni() const { return next<int>(); }
  istream *in;
};
Input in(cin);
class Output {
 private:
  ostream *out;
  template <typename T>
  void printSingle(const T &value) {
    *out << value;
  }

 public:
  Output(ostream &out) : out(&out) {}
  inline void setPrecision(int d) { *out << fixed << setprecision(d); }
  inline void print() {}
  template <typename T, typename... Ts>
  inline void print(const T &f, const Ts &...args) {
    printSingle(f);
    if (sizeof...(args) != 0) {
      *out << ' ';
      print(args...);
    }
  }
  template <typename... Ts>
  inline void println(const Ts &...args) {
    print(args...);
    (*out) << '\n';
  }
  template <typename... Ts>
  inline void operator()(const Ts &...args) {
    println(args...);
  }
};
Output out(cout);
namespace template_util {
constexpr int bytecount(uint64_t x) { return x ? 1 + bytecount(x >> 8) : 0; }
template <int N>
struct bytetype {};
template <uint64_t N>
struct minimal_uint : bytetype<bytecount(N)> {};
}  // namespace template_util
double EPS = 1e-10;
bool doubleEqual(double a, double b) { return fabs(a - b) < EPS; }
void solveOne() {
  auto n = in.ni();
  auto len = in.ni();
  vec<int> a(n + 2);
  a[0] = 0;
  a[n + 1] = len;
  for (int i = (1); i <= (n); ++i) a[i] = in.ni();
  int i = 0, j = n + 1;
  double t = 0;
  int s1 = 1, s2 = 1;
  double x1 = 0, x2 = len;
  out.setPrecision(15);
  while (i + 1 < j) {
    double d1 = a[i + 1] - x1;
    double t1 = d1 / s1;
    double d2 = x2 - a[j - 1];
    double t2 = d2 / s2;
    if (doubleEqual(t1, t2)) {
      t += t1;
      ++s1;
      ++s2;
      ++i;
      --j;
      x1 = a[i];
      x2 = a[j];
      if (i == j) {
        out(t);
        return;
      }
    } else if (t1 < t2) {
      t += t1;
      ++s1;
      ++i;
      x1 = a[i];
      x2 -= t1 * s2;
    } else {
      t += t2;
      ++s2;
      --j;
      x2 = a[j];
      x1 += t2 * s1;
    }
  }
  double d = x2 - x1;
  double s = s1 + s2;
  t += d / s;
  out(t);
}
void solve(istream &inStream, ostream &outStream) {
  in = Input(inStream);
  out = Output(outStream);
  auto tests = in.ni();
  while (tests--) {
    solveOne();
  }
}
int main() {
  solve(cin, cout);
  return 0;
}
