class Grade extends Student {
   
   private int score;
    
   public Grade( String firstName, String lastName, int phone, int score ) {
		super( firstName, lastName, phone );
		this.score = score;
   }
    
   public char calculate() {
       if ( score < 40 ) return 'D';
       else if ( score < 60 ) return 'B';
       else if ( score < 75 ) return 'A';
       else if ( score < 90 ) return 'E';
       else return 'O';
   }
   
}

Add day 12 solution