getting error:

Traceback (most recent call last):

File "python", line 26, in

File "python", line 22, in grades_variance

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

the instruction had me add float for len(grades) earlier which i didn't perfectly understand, while i try to play around with int and float in the last line but still have no idea what to do. anyone can offer insights? thanks.

'''

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):

average = grades_average(scores)

variance = 0

for score in scores:

variance += (average - scores)**2

`return (variance / float(len(scores)))`

print grades_variance(grades)

'''