The ISBN uses a weighted sum of the first nine digits in the number to create the check...

The ISBN uses a weighted sum of the first nine digits in the number to create the check digit, which is used to detect single-digit errors or the transposition of two digits. Each digit is weighted, by multiplying it by a constant, according to its position in the number. The weighted sum is found by the following formula:

The check digit is the remainder when the weighted sum is divided by 11. In the case of a remainder of 10, the check digit is an “X.”

Calculate the check digit for each of the following partial ISBNs.

a. 2-09-202101

b. 0-06-028548

c. 1-87-760346

d. 0-87-580218