To be more complete: .1a + .01a + ... + (digit sum / 9) will give it to you. That's actually what this method is doing, it's just poorly explained. 22103/9 =

To be more complete: .1a + .01a + ... + (digit sum / 9) will give it to you. That's actually what this method is doing, it's just poorly explained. 22103/9 = 2210 + 221 + 22 + 2 + (2+2+1+0+3)/9 = 2455 + 8/9 So your ones digit ends up being 2+2+1+0+(any whole part left over from the digit sum), your tens digit is 2+2+1, your hundreds digit is 2+2, and your thousands digit is just 2.