Wake Up Your Brain!!

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
a bus has no legs. There is no bus driver.

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.

Question: How many legs are there in the bus?

Note: Exilian got the right answer. Do you math before you peek.