Are you smarter than a 5th grader?

Caohien and I just spent half an hour or maybe more trying to figure this out.....and yes...we are smarter....ARE YOU?

There are 7 girls on a bus
Each girl has 7 backpacks
Each backpack has 7 big cats
For every big cat there are 7 little cats....

So how many legs are there in the bus?

Email me or google the answer.....
Here's Caohien's method of coming up with the anwer.