i am getting following errors,not able to understand what is wrong

B

Traceback (most recent call last):

File "python", line 52, in

File "python", line 48, in get_class_average

File "python", line 28, in get_average

TypeError: string indices must be integers, not str

not sure what is wrong in 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]
}
students = ["lloyd","alice","tyler"]
# Add your function below!
def average (numbers):
num =[31,23,23,45,0]
total = sum(numbers)
total = float(total)
total =total/len(numbers)
return total
def get_average(students):
homework=average(students["homework"])
quizzes=average(students["quizzes"])
tests=average(students["tests"])
return 0.1*homework+0.3*quizzes+ 0.6*tests
def get_letter_grade(score):
if score >= 90 and score <=100:
return "A"
elif score >= 80 and score <90:
return "B"
elif score >= 70 and score <80:
return "C"
elif score >=60 and score <70:
return "D"
else:
return "F"
print get_letter_grade(get_average(lloyd))
def get_class_average(students):
results = [ ]
for name in students:
results.append(get_average(name))
print results
return average(results)
print get_class_average(students)
print get_letter_grade(get_average(students))
```