Are you smarter than a 5th Grader?

This is a 5th grade math problem.     

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 either so don't worry about counting those legs!  That's all the help you get.   Here is the problem: 

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?

Tom Larsen