Okay are you smarter than a 5th grader....?

This is a 5th grade math problem. If you can open the spreadsheet, you'll see it's a very small list of people who have gotten the correct number. This is not a trick question. This is a real math problem so don't say that the bus doesn't have legs.

There are 7 girls in a bus. Each girl has 7 backpacks. In each backpack, there are 7 big cats. For every big cat, there are 7 little cats.

The bus driver is not in the bus at this time.

Question:
How many legs are there in the bus? (The number of legs is the password to unlock the excel spreadsheet. If you open it, add your name, save it, and then send it on to see who else can unlock it.)