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Coding challenge: Credit card number validator using Luhn's algorithm.

The process to validate a credit card number is:

1) put the card number into a reverse order 2) for every other number starting at the second digit, double that number's value. eg: 1 5 2 3 becomes 1 10 2 6 3) add these numbers together, treating 2 digit numbers as two separate values. eg: 1 + 1 + 0 + 2 + 6 4) if the credit card number is valid the total result should be a multiple of 10 eg: 50, 60 , 70 etc