Oops – forgot to mention one more thing: This “casting out nines” trick is used by bookkeepers all the time to help them track down errors. Let’s say your balance should be $12,768, but it keeps coming out to $12,345. The difference is $423 dollars. Take a look at the difference – is it evenly divisible by 9? 423 / 9 = 47 ---- yep, it’s divisible by 9. This means there’s a good chance that when you entered a number in your calculations, you transposed 2 of the digits! Think back to the explanation above – when you take any number, scramble it, and subtract one from the other, you always get a number that’s evenly divisible by 9). This can narrow down your search for the guilty number.