Mr.+D's+Homework+problem

How many legs????

7 girls get on a bus. Each girl has 7 backpacks. Each backpack has 7 cats. Each cat has 7 kittens. How many legs are on the bus? Show your answer so it is clear how you solved the problem from beginning to end. How would you prove your answer if somebody disagrees with you?


 * No tricks: The answer is NOT that the bus has wheels, not legs OR that you need to worry about the bus driver's legs. Just solve the problem.