7 girls each carrying 7 backpacks. So together there are 49 backpacks.

Each backpack has 7 big cats. So 49 backpacks have 49 x7 = 343 big cats

Each big cat is carrying 7 small cats(?) so, 343 cats will carry 2401 small cats.

Counting

7 girls will normally have 2 legs so 14 legs.

Each big cat will normally have 4 legs so 343(4) = 1372 legs

Each small cat will normally have 4 legs so 2401(4) = 9604 legs

Add them to get 9604 + 1372 + 14 = 10,990 legs in the bus.

I found an answer from www.scientificamerican.com

The Bicycle Problem That Nearly Broke Mathematics - Scientific ...

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I found an answer from www.quora.com

What is a brain teaser that is very short and extremely hard for adults ...

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I found an answer from www.splashlearn.com

What is Repeated Addition? - Definition, Facts & Example

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