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 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?