In creating a UPC code I have a number with 11 digits, and I need to generate
a 12th digit which is called a check character.
eg. 01234567890 _

Step 1) starting at the left add all characters in the odd positions (first
from the left, third from the left, ect.) eg. 0+2+4+6+8+0=20.

Step2) multiply the sum by 3. 20 x 3 = 60

Step 3) Starting from the left add all the characters in the even positions
(second from left, fourth from left, etc.) eg. 1+3+5+7+9 = 25

Step 4) Add the product of step 2 to the sum of step 3
60 + 25 = 85

Step 5) The check character is the smallest number which, when added to the
sum of Step 4, produces a multiple of ten.
Therefore the check character of the sample is - 5
85 + 5 = 90, a multiple of ten.