```
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 = 0.0
for grade in grades:
total += grade
return total
def grades_average(grades):
sum_of_grades = grades_sum(grades)
average = float(sum_of_grades)/len(grades)
return average
def grades_variance(scores):
average=grades_average(scores)
variance=0.0
for score in range(len(scores)):
variance= variance + (average-scores[score])**2
return variance/len(scores)
def grades_std_deviation(grades):
variance = grades_variance(grades)
return variance**0.5
print grades
print grades_sum(grades)
print grades_average(grades)
print grades_variance(grades)
print grades_std_deviation(grades)
```

I am getting a "Oops, try again. grades_std_deviation(515.5510204081633) raised an error: 'float' object is not iterable " error