Example 1:
Find the check digit: 0-11168-03004-?

Way #1:
0*3+1+1*3+1+6*3+8+0*3+3+0*3+0+4*3=46=6 mod 10;
The check digit is the number such that 6+N=0; so do 0-6=4.
The check digit is 4.

Alternately:
Digits in odd places: 0+1+6+0+0+4=11=1
1*3=3
Digits in even places: 1+1+8+3+0=13=3
Add them, and subtract from 0:
3+3=6
0-6=4
Check digit is 4

Example 2:

Find the check digit: 0-18008-01167-?

Way #1:
0*3+1+8*3+0+0*3+8+0*3+1+1*3+6+7*3=64=4 mod 10;
The check digit is the number such that 4+N=0; so do 0-4=6.
The check digit is 6.

Alternately:
Digits in odd places: 0+8+0+0+1+7=16=6
6*3=8
Digits in even places: 1+0+8+1+6=16=6
Add them, and subtract from 0:
8+6=14=4
0-4=6
Check digit is 6

Printable answers: