Hi!

I stuck at UNIT 9: EXAM STATISTICS:

This is my code

```
grades = [100, 100, 90, 40, 80, 100, 85, 70, 90, 65, 90, 85, 50.5]
def print_grades(scores):
for score in scores:
print score
print_grades(grades)
def grades_sum(scores):
total = 0
for score in scores:
total += score
return total
grades_sum = grades_sum(grades)
print grades_sum
def grades_average(scores):
sum_of_grades = grades_sum(scores)
average = sum_of_grades / float(len(scores))
return average
def grades_variance(scores):
average = grades_average(scores)
variance = 0
for score in scores:
variance += (average - score) ** 2
return variance / len(scores)
print grades_variance(grades)
def grades_std_deviation(variance):
return variance ** 0.5
variance = grades_variance(grades)
print grades_std_deviation(variance)
```

and this is output

```
100
100
90
40
80
100
85
70
90
65
90
85
50.5
1045.5
Traceback (most recent call last):
File "python", line 30, in <module>
File "python", line 24, in grades_variance
File "python", line 19, in grades_average
TypeError: 'float' object is not callable
```

can't get what's wrong ((( help me, please