Are you smarter than a 5th grader?

My brother-in-law sent me this via e-mail. See if you can figure it out.

This is not a trick question. This is a real math problem so don’t say that a bus has no 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

Question: How many legs are there in the bus?