Android. How to make some changes in Main class for the second class

Android. How to make some changes in Main class for the second class

/Hello, please help me to solve this problem. I am making an application
like "Millionaire"
I have made two Classes and two XML files for them. In the first class -
the first question, and in the second class - the second questions. in
both classes I made the button of 50:50(as in Millionaire). And I want to
make the 50:50 button in the second class not visible if I have already
used it in the first class./
public void onClick(View v){
Button btn50 = (Button) findViewById(R.id.btn50);
btn50.setVisibility(View.GONE);}
/btn50 is the Button of the second class and I am writing the code in the
first class, but when I try by this version and go from the first class to
the second one the application "unfortunately stops working"/
Please help me to solve this problem