class SimpleGradebook:
    def __init__(self):
        self._grades = {}
        
    def add_student(self, name):
        self._grades[name] = []
        
    def report_grade(self, name, score):
        self._grades[name].append(score)
        
    def average_grade(self, name):
        grades = self._grades[name]
        return sum(grades) / len(grades)