how to get random quiz question from and add listener on end of questions in quiz

I am teacher and creating a quiz for my students rest is complete I have 500 questions on my firebase real time database all I want to do is randomize questions each time 50 questions and set a listener after 50 questions here my quiz activity

public class grandtest extends AppCompatActivity {

TextView mQuestionTextView;
ProgressBar mProgressBar;
Button bchoice1,bchoice2,bchoice3,bchoice4;
Random Rand;
String mAnswer;
Timer mTimer;
TextView mScoreTextView;
public static int mScore;
int mQuestionNo = 0;

private Firebase mQuestionRef;
private Firebase mChoice1Ref;
private Firebase mChoice2Ref;
private Firebase mChoice3Ref;
private Firebase mChoice4Ref;
private Firebase mAnswerRef;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_grandtest);

    mQuestionTextView = findViewById(R.id.mquestiontextview);
    mProgressBar = findViewById(R.id.progressBar);
    bchoice1 = findViewById(R.id.choice1);
    bchoice2 = findViewById(R.id.choice2);
    bchoice3 = findViewById(R.id.choice3);
    bchoice4 = findViewById(R.id.choice4);
    mScoreTextView = findViewById(R.id.mScore);
}


public void updateQuestion (){

    mQuestionRef = new Firebase("https://class9notes-2808b.firebaseio.com/"+mQuestionNo +"/question");
    mQuestionRef.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
        String question = dataSnapshot.getValue(String.class);
        mQuestionTextView.setText(question);
        }

        @Override
        public void onCancelled(FirebaseError firebaseError) {
            Toast.makeText(grandtest.this,"Please enable data",Toast.LENGTH_SHORT).show();
        }
    });
    mChoice1Ref = new Firebase("https://class9notes-2808b.firebaseio.com/"+mQuestionNo+"/choice1");
    mChoice1Ref.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            String choice1 = dataSnapshot.getValue(String.class);
            bchoice1.setText(choice1);
        }

        @Override
        public void onCancelled(FirebaseError firebaseError) {

        }
    });
    mChoice2Ref = new Firebase("https://class9notes-2808b.firebaseio.com/"+mQuestionNo+"/choice2");
    mChoice2Ref.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            String choice2 = dataSnapshot.getValue(String.class);
            bchoice2.setText(choice2);
        }

        @Override
        public void onCancelled(FirebaseError firebaseError) {

        }
    });
    mChoice3Ref = new Firebase("https://class9notes-2808b.firebaseio.com/"+mQuestionNo+"/choice3");
    mChoice3Ref.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            String choice3 = dataSnapshot.getValue(String.class);
            bchoice3.setText(choice3);
        }

        @Override
        public void onCancelled(FirebaseError firebaseError) {

        }
    });
    mChoice4Ref = new Firebase("https://class9notes-2808b.firebaseio.com/"+mQuestionNo+"/choice4");
    mChoice4Ref.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            String choice4 = dataSnapshot.getValue(String.class);
            bchoice4.setText(choice4);
        }

        @Override
        public void onCancelled(FirebaseError firebaseError) {

        }
    });
    mAnswerRef = new Firebase("https://class9notes-2808b.firebaseio.com/"+mQuestionNo+"/answer");
    mAnswerRef.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            mAnswer = (String) dataSnapshot.getValue(String.class);
        }

        @Override
        public void onCancelled(FirebaseError firebaseError) {

        }
    });
    mQuestionNo ++;

I want to randomize mQuestionNo object to retrieve random questions each time. Here is structure of my firebfarebase dataase data