Khan Academy

Supporting grades: 6

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.

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