It the rule of transposition:
I know I'm a math geek, but I actually KNOW this, I didn't look it up. This will work with ANY number or digits.
When you take a number:
123
and jumble it up:
231
then subtract the two:
108,
the digits will ad up to a number divisible by 9:
1 + 0 + 8 = 9
You pick the 1, so you tell the guy 08 (leaving out the chosen number 1). He uses the single digit that will make your 0 + 8 = a number divisible by 9...in this case 9. He tells you its 1.
So you put in say 9876
Jumble: 6789
Subtract: 3087
You pick: 7
Tell him: 308
3 + 0 + 8 = 11 ..... 18 is the next number divisible by 9..... 18 - 11 = 7.
Yup, I'm a geek.
Cheers,
Dixie