Are YOU Smarter then a 5th Grader?

There are 7 girls on 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 on the bus?
This is a real math problem so don't say that the bus has no legs.

So tell me what your guess is and I'll reveal the answer later on...