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# I do not understand the code at 5:48

I am trying to figure out what the code does and I dont understand how it knows what week to read first. I am confused so if someone could explain this code step by step that would really give me a lot of clarity.

```travel_expenses = [
[5.00, 2.75, 22.00, 0.00, 0.00],
[24.75, 5.50, 15.00, 22.00, 8.00],
[2.75, 5.50, 0.00, 29.00, 5.00]
]

print("Travel Expenses:")
week_number = 1
for week in travel_expenses:
print("* Week #{}: \${}".format(week_number, sum(week)))
```

First of all to answer your question on how does it know which week to read.

If you type, len(travel_expenses), you will get 3. The reason why you will get 3 is because there are 3 lists inside of travel_expenses travel_expenses[0] would be [5.00, 2.75, 22.00, 0.00, 0.00], travel_expenses[1] would be [24.75, 5.50, 15.00, 22.00, 8.00], .... and on. for week in travel_expenses: ----- week is the new variable and it is going through all the stuff that are inside of travel_expenses, by all the stuff I mean the 3 lists that are inside it. So, when you first run the code, it runs through travel_expenses[0],which is [5.00, 2.75, 22.00, 0.00, 0.00], and it sums the total, then it runs through travel_expenses[1], which is [24.75, 5.50, 15.00, 22.00, 8.00], and it sums the total, and finally it does the last one travel_expenses[2], which is [2.75, 5.50, 0.00, 29.00, 5.00], and it sums the total.

Hope this helps a bit Good luck

Thank you very much

Hi, thank you for the question and the answer! Related follow-up question: why don't we have to define week? Is the for...in loop special in a way, so that you don't have to define the variable that follows for?

I'd appreciate some help :)

It's a default counter variable, which does not have to be defined in Python.