There are seven girls = 7 girls
These seven girls have seven bags apiece = 49 backpacks
In each backpack there are seven big cats = 343 big cats
And for each big cat there are seven small cats = 2401 small cats
Now we must find out how many legs are on the bus.
Assuming that each girl has two legs, and each cat has four,
7(2) + 343(4) + 2401(4)
So there are 10990 legs on the bus.