The Cats Turn

Solve this word puzzle:

ARE YOU SMARTER THAN A 5TH GRADER?

There are 7 girls on 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? The number of legs is the password to unlock the attached Excel sheet. If you open it, add your name and send it on to see who else can unlock it. Send it back to let the person know you got it correct .
P.S. There is NO driver

PS: The bad new? I can't put the Excel sheet address on here because it doesn't have one. You will have to comment or email me for answer. Some solve this quickly. Some don't.