my code is working fine but still they are not consedring it ryt ! help ! whats wrong in it?

```
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(scores):
variance=0
for score in scores:
t =(grades_average(grades)-score)**2
variance+=t
a=variance/float(len(scores))
return a
print grades_variance(grades)
```