Are you smarter than a 5th grader?

This is a real math problem so don't say that the bus has no legs. There are no tricks in the question. (There is no hidden bus driver, for example...)

ARE YOU SMARTER THAN A 5TH GRADER...?

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?
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