#include <bits/stdc++.h>
using namespace std;
long double it[100002];
long double xx = 0.00000000001;
long double pre[100002];
long double pre2[100002];
int n, l;
long double val;
bool check(long double mid) {
  long double c1 = 0;
  int co = 1;
  for (int i = 1; i <= n + 1; i++) {
    if (it[i] <= mid) {
      c1 += (it[i] - it[i - 1]) / co;
    } else {
      c1 += (mid - it[i - 1]) / co;
      break;
    }
    co++;
  }
  long double c2 = 0;
  co = 1;
  for (int i = n; i >= 0; i--) {
    if (it[i] >= mid) {
      c2 += (it[i + 1] - it[i]) / co;
    } else {
      c2 += (it[i + 1] - mid) / co;
      break;
    }
    co++;
  }
  val = c1;
  return c1 <= c2;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int t;
  cin >> t;
  while (t--) {
    ;
    cin >> n >> l;
    for (int i = 0; i < n; i++) {
      cin >> it[i + 1];
    }
    it[n + 1] = l;
    long double low = 0;
    long double high = l;
    for (int i = 0; i < 1000; i++) {
      long double mid = (low + high) / 2;
      if (check(mid)) {
        low = mid;
      } else {
        high = mid;
      }
    }
    check(low);
    cout << setprecision(10) << val << endl;
  }
  return 0;
}
