Come on Arend and Mart...

Its a string of numbers. You need to read it from right to left. Every 2nd digit you read, double it. If that value is greater than 10, say 14, add 1+4 to get 5, or if it was 16, add 1+6=7. Take these new single digits and stick them back into the original credit card number, and then add all the digits together. If the sum ends in 0, like 50 or 60 or 70, then its a valid credit card. Then you just have to check and make sure it is the proper number of digits and is a card your company accepts.