grades = [100, 100, 90, 40, 80, 100, 85, 70, 90, 65, 90, 85, 50.5]

def print_grades(grades):
    for grade in grades:
        print grade

def grades_sum(grades):
    total = 0
    for grade in grades:
        total += grade
    return total

def grades_average(grades):
    sum_of_grades = grades_sum(grades)
    average = sum_of_grades / float(len(grades))
    return average

def grades_variance(grades):
    average = grades_average(grades)
    variance = 0
    for grade in grades:
        grade = (average - grade) ** 2
        variance += grade
        wariancja = variance/len(grades)
    return wariancja
print grades_average(grades)