C#-level limits: How to load scenes randomly in 10 levels

i am making an alphabet game. i have 52 scenes (Easy mode of A-Z & hard mode of A-Z) but my game should only have 10 levels. level 1 to 5 is the easy mode. level 6 - 10 is the hard mode.
if the player reaches level 5, the next scene should be in random of the hard mode scenes which is 26-51.

this is my code so far:

using UnityEngine;
using System.Collections;

public class LevelTrigger : MonoBehaviour {

	int[] levelNumbers = new int[52]{0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51 };
	
	
	void OnTriggerEnter2D (Collider2D c){
		int randomIndex = Random.Range (0, levelNumbers.Length);
		Application.LoadLevel (levelNumbers [randomIndex]);
		return;
	}

}

what it does is that even in the 2nd level,it chooses from the hard mode set.

Create a second Array and split them between the easy and hard modes. If the next difficulty level is greater than 5, pull from the hard mode array instead of the easy mode array.