Simple instructional example for the basics of using event-driven programming. In real life, I would improve this code with lots of binding.

AdditionTest.java

import java.util.Random;

import javafx.application.Application;
import javafx.geometry.Pos;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.control.TextField;
import javafx.scene.layout.HBox;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class AdditionTest extends Application {

    private int currentQuestion = 0 ;
    
    private int currentCorrect = 0 ;
    
    private int firstNumber = 0 ;
    private int secondNumber = 0 ;
    
    private final Random rng = new Random();
    
    private final static int NUM_QUESTIONS = 5 ;
    
	@Override
	public void start(Stage primaryStage) {
		Label questionLabel = new Label();
		Label scoreLabel = new Label();
		
		TextField userEntry = new TextField();
		
		Button startButton = new Button("Start");
		Button submitButton = new Button("Submit");
		submitButton.setDisable(true);
		
		HBox questionBox = new HBox(5, questionLabel, userEntry);
		questionBox.setAlignment(Pos.CENTER);
		HBox buttons = new HBox(5, startButton, submitButton);
		buttons.setAlignment(Pos.CENTER);
		
		VBox root = new VBox(10, questionBox, scoreLabel, buttons);
		root.setAlignment(Pos.CENTER);
		
		startButton.setOnAction(event -> {
		    currentQuestion = 0 ;
		    currentCorrect = 0 ;
		    startButton.setDisable(true);
		    submitButton.setDisable(false);
		    makeQuestion(questionLabel, startButton, submitButton); 
		    userEntry.requestFocus();
		});
		
		submitButton.setOnAction(event -> {
		    try {
		        int answer = Integer.parseInt(userEntry.getText());
		        if (answer == firstNumber + secondNumber) {
		            currentCorrect++ ;
		        }
		    } catch (NumberFormatException exc) {
		        System.out.println("Couldn't convert "+userEntry.getText()+" into a number");
		    }
                    userEntry.setText("");
                    userEntry.requestFocus();
                    scoreLabel.setText(currentCorrect + " / " + currentQuestion + " correct");
                    makeQuestion(questionLabel, startButton, submitButton);
		});
		
		Scene scene = new Scene(root, 400, 400);
		primaryStage.setScene(scene);
		primaryStage.show();
		
		startButton.requestFocus();
	}
	
	private void makeQuestion(Label label, Button startButton, Button submitButton) {
	    currentQuestion++ ;
	    if (currentQuestion <= NUM_QUESTIONS) {
	        firstNumber = rng.nextInt(10)+10 ;
	        secondNumber = rng.nextInt(10)+10 ;
	        label.setText(firstNumber + " + " + secondNumber + " = ");
	    } else {
	        label.setText("");
	        submitButton.setDisable(true);
	        startButton.setDisable(false);
	    }
	}

	public static void main(String[] args) {
		launch(args);
	}
}