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

def grades_sum(scores):
    return sum(scores)

print grades_sum(grades)

def grades_average(grades):
    
    return grades_sum(grades)/float(len(grades))
print grades_average(grades)