From: Mark Day <mday@...>
Date: 2007-04-29T14:14:04+09:00
Subject: Re: [QUIZ] Checking Credit Cards (#122)

--Apple-Mail-1--199527535
Content-Transfer-Encoding: 7bit
Content-Type: text/plain;
	charset=US-ASCII;
	delsp=yes;
	format=flowed

On Apr 28, 2007, at 8:04 PM, Ari Brown wrote:

>> For example, given the card number 4408 0412 3456 7893:
>>
>> 	Step 1:  8 4 0 8 0 4 2 2 6 4 10 6 14 8 18 3
>> 	Step 2:  8+4+0+8+0+4+2+2+6+4+1+0+6+1+4+8+1+8+3 = 70
>> 	Step 3:  70 % 10 == 0
>>
>> Thus that card is valid.
>
> Uh, this is probably just affecting me but....
>
> In his example, after he doubles the second to last digit (call it  
> d), he uses mod10 on it (9*2 = 18 %10 = 8). That is the way to get  
> his numbers, but a) where does he say that and b) where do the 10  
> and 14 come from?

This confused me at first, too.  Let's take just the last four  
digits: 7893.  Counting from the end, double every second digit,  
leaving the others unchanged.  That gives you: 14, 8, 18, 3 (where  
14=7*2 and 18=9*2; the 8 and the 3 are unchanged). Now add up every  
digit of the resulting sequence: 1+4  +  8 +  1+8  +  3.  Note the "1 
+4" comes from the 14, and the "1+8" comes from the 18.

Does that help?

-Mark


--Apple-Mail-1--199527535--