Exercise solution:
Course and Project classes

using System;
using System.Collections.Generic;

public abstract class Course{      

  private string name;

  public Course(string name){      
    this.name = name;
  }

  public abstract bool Passed();

}

public class BooleanCourse: Course{

  private bool grade;

  public BooleanCourse(string name, bool grade): base(name){
    this.grade = grade;
  }

  public override bool Passed(){
    return grade;
  }

}

public class GradedCourse: Course{ 

  private int grade;

  public GradedCourse(string name, int grade): base(name){
    this.grade = grade;
  }  

  public override bool Passed(){
    return grade >= 2;
  }

}

public class Project{

  private List<Course> courses;     // Now a list of courses instead of
                                    // four variables of type Course

  public Project(Course c1, Course c2, Course c3, Course c4){
                                    // Could/should be generalized to one parameter of type List<Course>
    courses = new List<Course>();  

    courses.Add(c1); courses.Add(c2);
    courses.Add(c3); courses.Add(c4);
  }

  public bool Passed(){             // Reimplemented. Counts the number 
                                    // of passed courses
    List<Course> passedCourses = 
      courses.FindAll(delegate(Course c){return c.Passed();});

    return passedCourses.Count >= 3;
  }

}

public class Program {

  public static void Main(){
    Course c1 = new BooleanCourse("Math", true),        
           c2 = new BooleanCourse("Geography", true),
           c3 = new GradedCourse("Programming", 2),
           c4 = new GradedCourse("Algorithms", 4);

    Project p = new Project(c1, c2, c3, c4);

    Console.WriteLine("Project Passed: {0}", p.Passed());
  }

}