Math problem

I got this match problem right on the first try. Can you?

FACTS:
There is a bus with 7 children inside of it
Each child has 7 rucksacks
In each rucksack there are 7 big cats
Every big cat has 7 small cats
Everything listed above is entirely in the bus
All objects are unique
There is no driver
Every child has 2 legs
Every cat has 4 legs

QUESTION:
How many legs are in the bus?

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