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?

Well, girls generally have two. Cats generally have 4. I have never seen a backpack with legs. Soooo, I would count the number of cats, multiply by 4 and then add the number of girls' legs.