"Oops, try again. One of the following is missing or broken when we tried to use it: alice, lloyd, tyler, students, get_class_average, get_letter_grade"

this is the error what does it mean???

the code :

lloyd = {

"name": "Lloyd",

"homework": [90.0, 97.0, 75.0, 92.0],

"quizzes": [88.0, 40.0, 94.0],

"tests": [75.0, 90.0]

}

alice = {

"name": "Alice",

"homework": [100.0, 92.0, 98.0, 100.0],

"quizzes": [82.0, 83.0, 91.0],

"tests": [89.0, 97.0]

}

tyler = {

"name": "Tyler",

"homework": [0.0, 87.0, 75.0, 22.0],

"quizzes": [0.0, 75.0, 78.0],

"tests": [100.0, 100.0]

}

# Add your function below!

def average(numbers):

total=sum(numbers)

total=float(total)

return total/len(numbers)

def get_average(student):

homework=average(student["homework"])*0.10

quizzes=average(student["quizzes"])*0.30

tests=average(student["tests"])*0.60

return homework+quizzes+tests

def get_letter_grade(score):

if score>=90:

return 'A'

elif score>=80:

return 'B'

elif score>=70:

return 'C'

elif score>=60:

return 'D'

else:

return 'F'

def get_class_average(students):

results=[]

for item in students:

x=get_average(item)

results.append(x)

x=average(results)

return x

print get_class_average([lloyd,alice,tyler])

print get_letter_grade(get_class_average([lloyd,alice,tyler]))