First class(StartGame.java):
-----------------
package com.Slimenian.ChatGames;

import java.util.Random;

import org.bukkit.command.Command;
import org.bukkit.command.CommandExecutor;
import org.bukkit.command.CommandSender;



public class StartGame implements CommandExecutor
{

    @Override
    public boolean onCommand(CommandSender sender, Command cmd, String str, String[] args)
    {
        if(args.length > 0)
        {
            if(args[0].equalsIgnoreCase("math"))
            {
                math(sender);
                return false;
            }
        }

        help(sender);

        return false;
    }

    //NAJ ŠTEJE TOČKE, KO PLAYER DOBI 10 PRAVILNIH ODGOVOROV DOBI 100(AL PA MAL VEČ)$


    void help(CommandSender sender)
    {
        sender.sendMessage("/ch math");
    }

    public void math(CommandSender sender)
    {
        Random rand = new Random();
        double a = rand.nextInt(10)+ 1;
        double b = rand.nextInt(10)+ 1;
        int operator = (int)rand.nextInt(4);

        char op = ' ';
        switch(operator)
        {
        case 0: op = '+'; break;
        case 1: op = '-'; break;
        case 2: op = '*'; break;
        case 3: op = '/'; break;
        }


        double odg = c(operator, a, b);


        sender.sendMessage(String.valueOf(a)+ " " + Character.valueOf(op)+ " " +String.valueOf(b));
        sender.sendMessage(String.valueOf(c(operator,a,b)));

    }

    public double c(int operator,double a,double b)
    {
        double c = a;
        switch(operator)
        {
        case 0: c += b; break;
        case 1: c -= b; break;
        case 2: c *= b; break;
        case 3: c /= b; break;
        }
        return c;
    }


}


--------------------
Second class(ChatListener):

package com.Slimenian.ChatGames;

import org.bukkit.event.EventHandler;
import org.bukkit.event.Listener;
import org.bukkit.event.player.AsyncPlayerChatEvent;

public class ChatListener implements Listener
{
    @EventHandler
    public void onChat(AsyncPlayerChatEvent evt)
    {
        if(evt.getMessage().equalsIgnoreCase(String.valueOf(c)))
        {

        }
    }
}

---------------
Third class(Main.java):

package com.Slimenian.ChatGames;


import org.bukkit.Bukkit;
import org.bukkit.plugin.PluginManager;
import org.bukkit.plugin.java.JavaPlugin;


public class Main extends JavaPlugin
{
    PluginManager pm = Bukkit.getPluginManager();


    @Override
    public void onEnable()
    {
        getLogger().info("Plugin Enabled");
        this.getCommand("ch").setExecutor(new StartGame());
        Bukkit.getPluginManager().registerEvents(new ChatListener(), this);
    }



    @Override
    public void onDisable()
    {
        getLogger().info("Plugin Enabled");
    }
}