Description: Why you can add the digits to see if something is divisible by 3. Created by Sal Khan. I can just add up the digits. If the sum of that is a multiple of 3 then this whole thing is a multiple of 3. So you say 4 plus 7 plus 9 plus 2. That's 11. If you're unsure, you can even add the digits of that 2 plus 2 is 4.*Teacher Advisor is 100% free.