Credit Card Number Check. The last digit of a credit card number
is the check digit, which protects against transcription errors
such as error in a single digit or switching two digits. The
following method is used to verify actual credit card number but,
for simplicity, we will describe it for numbers with 8 digits
instead of 16: Starting from the rightmost digit, form the sum of
every other digit. For example, if the credit card number is
43589795, then...