31. Create a check digit for the UPC number 38137009213 using the weights 7, 1, 7, 1, 7, 1, …, 7, 1, instead of 3, 1, 3, 1, 3, 1, …, 3, 1. Test to see whether this check digit will detect single-digit errors by trying several examples.
31.
The check digit is 7. This check digit method detects all single-digit errors.