class Exercise {
	private int studentAnswer;
	private char grade;

	private final String question;
	private final int modelAnswer;

	public Exercise(String q, int a) {question = q; modelAnswer=a;}

	public void writeAnswer(int anAnswer) {studentAnswer=anAnswer;}
	public void readAnswer() {System.out.println("Student's answer is "+studentAnswer);}
	public void grade() {if (studentAnswer==modelAnswer) grade='A';else grade='F';}

	public void displayResult() {System.out.println("Student's answer is "+studentAnswer+", grade is: "+grade);} 
}

class Student {
	private String name;
	public Student(String n) {name=n;}
	public void doExercise(Exercise x, int answer){x.writeAnswer(answer);}
}

class Grader {
	private String name;
	public Grader(String n) {name=n;}
	public void gradeExercise(Exercise x){x.grade();}
}

public class Main
{
	public static void main(String[] args) 
	{
		Exercise ex = new Exercise("What is 4!", 24);
	
		Student m = new Student("Mary");
		Grader h = new Grader("Helena");
	
		m.doExercise(ex,24);
		h.gradeExercise(ex);
	
		ex.displayResult();
	}
}