I've got this in an e-mail, it is very simple. The idea was an excel sheet that locked by a password. The password is the answer of the following mathematical question:

There is a bus with 7 girls.
Each girl has 7 bags.
In each bag, there are 7 big cats
Each big cat has 7 little cats.
Each cat has 4 legs.
Question: How many legs are present in bus?

It was said that few resolve it, however it is not that hard!