Cracking the Code

Credit card security has a secret code to validate cards. You start with a number then cross out every other one and double it. Then cross out any double digits and add them together. Then add all the numbers that are not crossed out together and if it is divisible (i don't know if that is the right word!) by 10 it is a good number! Seems a little complicated, but is it pretty cool that it works when you try it. MSN Money has a an article on it that includes a worksheet that you can download and fill out.