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diff --git a/mit/extented_class_example/GradeBook.py b/mit/extented_class_example/GradeBook.py
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+++ b/mit/extented_class_example/GradeBook.py
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+#!/usr/bin/python3
+
+from MITPerson import *
+
+class Grades(object):
+    """ A mapping from students to a list of grades """
+    def __init__(self):
+        """ Create empty grade book """
+        self.students = []      # List of student objects
+        self.grades = {}        # maps idNum -> list of grades
+        self.isSorted = True    # true if self.students is sorted
+
+    def addStudent(self, student):
+        """
+            Assumes: student is of type Student
+            Add student to the grade book
+        """
+        if student in self.students:
+            raise ValueError('Duplicated student')
+
+        self.students.append(student)
+        self.grades[student.getIdNum()] = []    # dict is a ID:List pair
+        self.isSorted = False
+
+    def addGrade(self, student, grade):
+        """
+            Assumes: grade is a float
+            Add grade to the list of grades for student
+        """
+        try:
+            self.grades[student.getIdNum()].append(grade)
+        except KeyError:
+            raise ValueError('Student not in grade book')
+
+    def getGrades(self, student):
+        """ Return a list of grades for student """
+        try:    # Return a COPY of student's grades
+            return self.grades[student.getIdNum()][:]
+        except KeyError:
+            raise ValueError('Student not in grade book')
+
+    def allStudents(self):
+        """ Return a list of the students in the grade book """
+        if not self.isSorted:
+            self.students.sort()
+            self.isSorted = True
+        return self.students[:] # Return a COPY of students, so we avoid any
+                                # externalmodification to the original list
+
+
+
+### TESTS
+
+def gradeReport(course):
+    """ Assumes: course is of type grades """
+
+    report = []
+
+    for s in course.allStudents():
+        tot = 0.0
+        numGrades = 0
+
+        for g in course.getGrades(s):
+            tot += g
+            numGrades += 1
+
+        try:
+            mean = tot/numGrades
+            report.append(str(s) + '\'s mean grade is ' + str(mean))
+        except ZeroDivisionError:
+            report.append(str(s) + ' has no grades')
+
+    return ' \n'.join(report)
+
+
+six00 = Grades()
+ug3 = UG('Drew Houston', 2017)
+ug1 = UG('Matt Damon', 2018)
+ug2 = UG('Ben Affleck', 2019)
+ug4 = UG('Mark Zuckerberg', 2017)
+g1 = Grad('Bill Gates')
+g2 = Grad('Steve Wozniak')
+
+six00.addStudent(g1)
+six00.addStudent(ug2)
+six00.addStudent(ug1)
+six00.addStudent(g2)
+six00.addStudent(ug4)
+six00.addStudent(ug3)
+
+six00.addGrade(g1, 90)
+six00.addGrade(g2, 45)
+six00.addGrade(ug1, 80)
+six00.addGrade(ug2, 75)