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all right in this video we're going to be talking about credit cards and their check digit all right so let's go ahead and get started all right so first off you have a credit card example now this information is just extra information i'm not necessarily going to test you on this but if whatever your credit card starts with this is what it could be so like for example if your credit card starts with a three it could be an american express diners club carte blanche i guess uh it starts with the four it could be it it's going to be a visa card if it's it starts with the five it's a mastercard and if it starts with six the discover card all right so that's just kind of like extra information but anyway from a mathematical standpoint the credit card makes up 16 digits all right and inside of these 16 digits there's actually a small little algorithm that we're going to discuss all right now if you want to see what makes up the credit card like the little history the makeup whatever then just pay attention to this down here the issue the issuer id number account number and uh there you go so i'm going to let you look at that on your own time but let's go ahead and just dive into the actual algorithm of a credit card all right now because the fact that i need a little more room to write i'm going to rewrite this credit card number um in a bigger format so one second all right there you go all right i don't know i told you i'll hold on i could easily just pause the video but all so what i did was i took uh jason uh jason's credit card and just made it a little bigger just it'll give us a little more room to write all right now here's the algorithm and it's four steps okay so the first step is to add all the odd position digits together okay that's our first step now the question is like what the heck is an odd position digits well odd position is like the first number the third number so number four the first number that is an odd position digit that's an odd position digit five the eight is an odd position now remember it's not odd numbers it's odd position like first place third place fifth place uh seventh place i believe that's ninth place that's 11 place notice how basically i'm just skipping i'm starting with the first number and skipping every other one now once you identify what they are you are going to add them all up so i'm going to add them all up here so 4 plus zero plus five plus uh let's see eight plus one plus zero again plus two plus eight all right so when i add all that up i think i can do it by hand but i'm probably going to mess up giving a video all right that ends up being 28 okay now it also says multiply by two so you're going to take the sum you're going to times that by 2 and you're going to get 56. so what is the total step of step one or what is the total for step one you would say it's 56. all right so once you get 56 and let me just go ahead and type it in here you're going to pause for a second all right so that 56 gets put on hold all right now let me go ahead and tell you step two is the more challenging step because it's just weird it's just straight out weird and that is using the digits from step one determine how many digits are actually more than four so in other words what the the numbers that we highlighted from step one remember everything in yellow here we've got to then analyze those digits to find out which ones are more than four which which means which one are in the five to nine range all right so five to nine range now it's as easy as looking okay this first number four is that five to nine no it's not we're not going to do anything with that zero nope oop five oh that works i'm just going to check it for now this eight yep this eight is one nope the zero nope this two nope but this eight there is now was challenging about this question is that it's so easy it's difficult so how many how many digits were in the five to nine range you don't add them up you just count one two three it was a total of three so we're going to say the total for step two is three now lastly add all the even position digits together basically what that means is all the numbers that you skipped just add them up all right so that means all the all the numbers like that two that three that's six that's seven everything that you haven't messed with go ahead and add it up so let's see um two plus three let's say that's five plus six that's 11 plus seven that is 18 plus zero still 18 plus nine i believe that's 27 um plus one is 28 and then 3 is 31. all right so all the other numbers added together is 31. now lastly what is the or i'm sorry step four add the total for each step so that means i'm going to add 56 3 31. all right so 56 plus 3 plus 31 when i add them all together i believe that's going to give me 100 even plus 3 plus 31. wow sorry 90 gave us 90. now it says is this a valid credit card and the answer is yes the reason why i know the answer is yes is because again very simple don't miss this it's because the last number of our digit i'm sorry the last number of our total was a zero all right so if the total is a multiple of 10 it is a valid you i'm sorry valid credit card all right so again what is the total it's 90. 90 is a multiple of 10 or either way it in or even easier it ends in zero it is a valid credit card now let's play the what if game what if you added all this up and it ended up being let's just say 91 well 91 is not a multiple of 10 or in other words keep it simple it doesn't end in zero so this would not be yes this would be a flat out no it would not be valid all right so next question brian works for liberty bank and trust as an account manager his client john smith started a new account brian created the new credit card answer the following questions regarding the credit card alright so using the credit card procedure so again the credit card procedure step one is to starting with the odd position digits now this is a little smaller i'm just gonna maybe i can do a highlighter here all right so i'm gonna start with four i'm gonna start with seven i'm gonna start with three or not start with but five there five and nine remember step one is to add all those numbers up and times it by two so four plus seven plus three plus five plus zero plus seven plus five plus nine times that by two i get eighty so for step one i'll just type it just to make everything neat here all right so step one ended up being 40. all right now for step two now remember step two is the weird one and that is the numbers that we chose it which one is in the five to seven range okay so which one is in the five to seven range all right that's always going to be the weird one so we'll do this in red here all right four no four is not in the five to seven range i'm sorry five to nine range seven is three no five yes no seven yes five yes nine yes one two three four five so that means for step two it's five for step three i add up all the numbers that i skipped originally which means this two this five this one this six this three this two is four and three add it all up it ends up being 26. and then lastly for step four very simply add up all the numbers from all the other steps so 40 plus 5 plus 26 that gives me 111. now remember the rule from uh you know five seconds ago and that is if the number is not a multiple of 10 which means in other words does the total end in the zero it's not a valid credit card so you would say because of the fact that it's 111 it is no so it is not a valid credit card all right now the last thing that we're going to talk about is the actual check digit okay so the check digit is let me go back to our first screen let me clean this up a little bit all right so at the very bottom here you see the last number is the check digit okay so the check digit is the number that makes the rest of these numbers accurate okay so when you're doing the credit when you're doing these procedures if you have a a correct check digit if the check digi is correct it's what validates the rest of the numbers which another what means it makes all the totals add up to zero or add up to a number that equals zero so what that means is like for example in this question right here someone should be getting fired right here someone should be getting fired because of the fact there's no way that this last number which is the check digit should be a three because the reason it shouldn't be a three is because everything added up to 111 the check digit is made so that would not happen okay now you might be scratching you're saying what the heck you're talking about well let's look at this all right so for this master card what we have here is the check digit which is unknown okay so let's say you're in charge of finding the check digit you're in charge of making sure that that number which we don't know makes the rest of these numbers valid okay so what you would do is the exact same thing as you would do with all the other with all the steps all right so again step one taking the first number skipping every other one adding them up and then times them by two so step one equals 52 okay it added up to 26 i doubled it 52. all right step two remember that's the crazy one that's the one that says any number that's in the five to nine range so let's see five yes that's one no seven yes that's two no three no one no three no three five yes so there's a total of three digits now when it comes to finding the check digit notice or well i'm sorry when it comes to step three sorry i jumped ahead step three is you're adding all the numbers that you didn't mess with all right so everything that you skipped now you might be asking wait a minute we have an x here how can i add x into that well this is what you do when you're finding the check digit i need you to almost like forget about it for a second all right so when you're finding the check digit i need you to almost forget that it's there and just add all the other numbers up all right so that means adding this 4 up this 2 this 5 is 2 this 4 this 2 and this 4. okay so when we do that step 3 ends up being let's see step 3 ends up being 23 okay which means step four when you add everything up adds up to being 78. now here's the thing all these numbers from the first number to the 15th number all of these numbers add up to 78 okay this check digit that we don't know is the number that makes 78 into valid into a valid total now remember in order for something to be valid it has to the total has to end in a zero so think to yourself if i'm sitting on 78 how many more how many more numbers do i have to add to it to make it the next valid number which is 80. and you would say well i just have to add 2. if i added 2 to my total then it would add up to get to 80 and then it would be valid so whatever you add with that would be the check digit so the check digit in this example would be two so your credit card number would be for the last number of the credit card would be two because that's what would make this total valid now you might be saying well okay hold on let's try one more all right so let's try one more again doing steps one two three four again step one i'm just going to go ahead and speed up a little bit step one is equal to 76. step 2 equals 5 step 3 would equal 31 so that means step four i don't know why keep on pressing that number or that key there step four adds up to 112. okay all right so we have an incomplete credit card number here because of the fact that we have 15 but we don't have a credit card we don't have our check digit so all of this added together the first 15 numbers added up to 112. well what has to be added to 112 to make it a valid credit card well if i'm sitting on 112 i know that the next valid total would be 120 which means i would have to add eight so the check digit would have to be eight because i would have to add that to the total so you'd say the check digit would be eight all right so there you have the check digit you have the credit card algorithm let me know if you have any questions thank you for watching