Here is my code:

```
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(x):
average = grades_average(x)
variance = 0
for i in x:
variance += (average - x) ** 2
variance / len(x)
return variance
print grades_variance(grades)
```

My error:

Traceback (most recent call last):

File "python", line 25, in

File "python", line 22, in grades_variance

TypeError: unsupported operand type(s) for -: 'float' and 'list'

Thanks.