Can someone please help me out with what is wrong with this code?

Why it keeps returning the error message; ‘grades_average([2, 4]) returned 80.42307692307692 instead of the expected: 3.0’

and how it is any different from this code;

def grades_average(grades):

sums = grades_sum(grades)

G_average = sums / float(len(grades))

return G_average

print grades_average(grades)

Thanks in advance