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chances are you have a credit card in your wallet on the front is usually something like your name and other information like your bank and the card carrier like Visa MasterCard etc and a string of sixteen fifteen or fourteen numerical digits these digits vary from card to card and they're enough for merchants to transact business with your bank on your behalf however this string of numbers isn't random in fact there's a secret code in there that validates your card and keeps it safe all in the number structure it's a low-tech mathematical algorithm called the loon formula invented by Hans Peter Luhn in 1954 this mathematical tool actually dates back to before the existence of credit cards even with that fact it's included in every credit debit an ATM card on earth and it also makes for a mind-blowing bar trick if people don't know it here's how you can see how it works first you'll want to write out your or any credit card number on a piece of paper this is a randomly generated Visa card number so it's not real you can try but it's not going to work now once you have that card number written out you'll double every other number starting with the second number from the right and then write them below that card number like this once you've done that you'll have a new string of numbers with every other number doubled starting from the right after doing that any two-digit numbers as a result of the doubling you'll sum both of those individual numbers together to get a single number so with our first two digit number of 16 you'll sum 1 in 6 to get 7 making a 1 digit number extrapolating that process out here's our new string finally at all the numbers you're left with in the string doubled and original like below ignoring the original numbers that you doubled and now have a new number 4 if your final sum is divisible by 10 then it's a valid card number if the final sum isn't divisible by 10 then the card number isn't valid or you did something wrong in the case of our sample number it's just not in use but it's still a valid number this algorithm is used to calculate every single card number on earth Visa Mastercard Discover they all have 16 digits and American Express has 15 and then diners club cards have 14 each different card issuer will have different subcategories of what numbers mean in their string like issuer ID account number etc but they all have to line up with the lewd algorithm back to the Loon algorithm itself the last number in your credit card is referred to as a check digit as the computer runs the algorithm it will complete its sum without the last number using our sample number as an example we've replaced the final digit of nine with an X...