Global Variable and operator - between two Global variables

Question

I am new to android studio and programming. I am facing an issue. I have decleared two Global variables here.

package com.example.letsbid;

public class GlobalVariable
{
    public static int balance = 100;
    private static int deduction=15;
}

Then I assign the value of that Global variable to a text view

tv_balance.setText(String.valueOf(GlobalVariable.balance));

now I want whenever user click on a specific button that balance should reduce 15 points. for example if it is 100 it should become 85, if its 60 it should become 45. For that I am trying this

         btn_515.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

              // error in here following code, i tried balance=balance-15;     
             // I tried balance=balance-deduction;
             // I tried balance==balance+""-deduction+"";
             balance==balance-deduction;

                tv_balance.setText(String.valueOf(GlobalVariable.balance));

                Intent intent = new Intent(BalanceActivity.this, LotteryActivity.class);
                startActivity(intent);
            }
        });

But it shows error operator - cannot be applied to Global variable

Now how can I apply that - to that so that everty time user click on btn_515, 15 should be deducted from his balance.