C#-Code

using System;
using static System.Console;

String balloon = "balloon";
String[] test = new String[] { "nlaebolko", "loonbalxballpoon", "balbalon" };
for (var i = 0; i < test.Length; i++)
    WriteLine($"{test[i].Solution(balloon)} x {balloon} in {test[i]}");
ReadKey();

static class Extension  {
    public static int Solution(this string ss, string balloon)  {
        char[] b = balloon.ToCharArray();
        char[] s = ss.ToCharArray();
        char nu = '\0';
        int counter = 0;
        bool bo_ready = false;
        while (!bo_ready)  {            
            for (var i = 0; i < b.Length; i++)  {
                bo_ready = true;
                for (var j = 0; j < s.Length; j++)
                    if(b[i] == s[j])  {
                        counter++;
                        s[j] = nu;
                        bo_ready = false;
                        break;
                    }
                if (bo_ready)
                    break;
            }
        }        
        return counter / b.Length;
    }
}