How can i make and calculate a new List that will have the items minus the number of items from another List ?

Another way:

using System;
using System.Collections.Generic;
 
namespace CSharp
{
    class Program
    {
      
        static void Main()
        {
           //Generate for List
            List<string> file = new List<string>();
            for (int i = 0; i < 61; i++)
            {
                file.Add(string.Format("file{0:000000}", i));
            }
 
            //Copy a copy list
            List<string> copyfile = new List<string>(file);
 
            //Generate for List1
            List<string> file1 = new List<string>();
            for (int i = 0; i < 9; i++)
            {
                file1.Add(string.Format("test{0:000000}", i));
            }
 
            //Check file
            for (int i = 0; i < file.Count; i++)
            {
                if(file1.FindIndex(s=>s.Substring(4)==file[i].Substring(4))>=0)
                {
                    //Do delete
                    file.RemoveAt(i);
                    --i;
                }
            }
 
            //Output
            foreach (var item in file)
            {
                Console.WriteLine(item);
            }
        }
    }
}

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