19. When calculating the check digit for a 13-digit ISBN, why can you disregard the first two digits? (Try it for Exercise 16.)
19.
The lead digits 97 contribute 30 to the weighted sum. Thus, the digit needed to make the sum evenly divisible by 10 is the same if you leave the 9 and 7 out of the calculation. (For example, if the sum were 162, including the 97 digits, then the sum would be 132 if it didn’t include them. In either case, the check digit is 8.)