Riddle - can you solve ?

Hey, wait a minute! I was the first to give the correct answer!!!

Go on then so, you can sit up at the desk beside me, no messin' outta ya now
 
It says they have 7 backpacks but it doesn't say the backpacks are in the bus.

Of course it does !


There are 7 girls in a bus.
Each girl has 7 backpacks (on their backs on a bus).
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?