How do i apply a value to my 'grade' variable?

Here's the code -

def calc_average():
   avg = sum(list) / len(list)
   if avg >= 80:
     grade = 'A' 
   elif avg >= 75 < 80:
     grade = 'B+'
   elif avg >= 70 < 75:
     grade = 'B'
   elif avg >= 65 < 70:
     grade = 'C+'
   elif avg >= 60 < 65: 
     grade = 'C'
   elif avg >= 55 < 60:
     grade = 'D+'
   elif avg >= 50 < 55:
     grade = 'D'
   else:
     grade = 'F'
   print('Your average is:', avg, '% and your grade is', grade)

How do I assign a value to my 'grade' variable because when I print it out it says

local variable 'grade' value is not used

or am I just doing it wrong...