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How to validate a credit card number mentally



Do you know you can find out whether a credit card number is valid or not just by simple mental mathematics and you don’t need complicated programs to verify the validity of a number. Of course, if the credit card is inactive or defunct it is another matter, but if you just want to check whether the number present on the credit card is valid or not you don’t need a machine or a computer to tell you that. In terms of technique, here is what you have to do

  • Starting from the right, double (multiply the number by two) every second digit and note it down
  • In case after doubling the digits you get double digits answers (something like 10 or 12) you can write them as 1 0 or 1 2 or 1 3
  • Add these doubled digits to the digits that were not doubled and then add everything up
  • If the result is divisible by 10 then your credit card number is valid otherwise it is invalid

OK, I know without an example it may seem a bit difficult, so let us test this out with an example. Let us use tabular data to properly understand the numbers involved. The top row in the table presented below is the original credit card number (of course it shouldn’t be working). The second row is the alternate digits doubled. The third row converts every double digit into single digit and the third row adds up all the digits:

4
4
7
6
9
7
5
3
8
2
4
0
4
1
5
3
8
14
18
10
16
8
8
10
8   1+4   1+8   1+0   1+6   8   8   1+0  
8+ 4+ 5+ 6+ 9+ 7+ 1+ 3+ 7+ 2+ 8+ 0+ 8+ 1+ 1+ 3

And the final number comes out to be 73, which, of course, is not divisible by 10 and hence this is not a valid credit card number. In case you have access to a valid credit card number you can try it out yourself.