This method, also known as modulo 10, is used to verify credit card numbers. It can verify if adjacent digits have been swapped or misread.
Starting with the rightmost digit, add up all odd digits, then, for each even digit, multiply by 2 and cast 9 (add digits together when > 9), then add to the sum. This total sum must be an even modulo 10.
 6   1   8   2        0   9   2   3        1   5   5   3 (bolded is x2)
12   1  16   2        0   9   4   3        2   5  10   3 (reduce 2 digits)
 3   1   7   2        0   9   4   3        2   5   1   3 (add em up)
Total 40 mod 10 = 0 - GOOD!
Canadian SIN version is almost identical, but the final result is rounded up to the next mod 10 and the difference should equate the last digit.

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