Card number: the last digit

The problem

Bank card numbers end in a check digit (the Luhn method). Write a function cardCheckDigit(first15) that gets the first 15 digits as text and gives back the last digit as a number. How: start from the RIGHT of first15. Double the 1st, 3rd, 5th ... digit from the right; if a doubled value is more than 9, subtract 9. Add everything up. The check digit is (10 - total % 10) % 10. (These are made-up test numbers, not real cards.)

You will practice: check digits, Luhn method, loops from the right, remainder (%)