#include <bits/stdc++.h>
using namespace std;
double n, t, l, a[100005];
int main() {
  cin >> t;
  while (t--) {
    cin >> n >> l;
    for (int i = 1; i <= n; i++) cin >> a[i];
    int i1 = 1, i2 = n;
    double speed1 = 1, speed2 = 1, t1 = 0, t2 = l, ans = 0;
    while (1) {
      if (i1 > i2) {
        printf("%.7lf\n", ans + (t2 - t1) / (speed1 + speed2));
        break;
      } else if ((a[i1] - t1) / speed1 == (t2 - a[i2]) / speed2) {
        double time = (a[i1] - t1) / speed1;
        ans += time;
        t2 = a[i2], t1 = a[i1];
        i1++;
        i2--;
        speed1++;
        speed2++;
      } else if ((a[i1] - t1) / speed1 < (t2 - a[i2]) / speed2) {
        double time = (a[i1] - t1) / speed1;
        ans += time;
        t2 = t2 - time * speed2, t1 = a[i1];
        i1++;
        speed1++;
      } else {
        double time = (t2 - a[i2]) / speed2;
        ans += time;
        t1 = t1 + time * speed1, t2 = a[i2];
        i2--;
        speed2++;
      }
    }
  }
}
