The Self-Effacing Number 9

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Transcript:
"Here's a bonus, it's called the "self-effacing nine." This is very helpful if you're working with numerical data in columns. Like you're doing bookkeeping, or you're an accountant, or perhaps you're doing your taxes, or your bank reconciliation, and debits and credits have to match up. Any time you're out, it's off, it's out of balance by a number divisible by nine, you reverse the number.

I'll give you two examples. Let's do an easy one. Your adding up thirteen, and twenty three, and twelve. This adds up to eight, forty eight. Let's say on your other side with your debits and credit, You think you've written it down the same way, and it adds up to sixty six. Is that correct? Sixty six. Well that's not the same and you wonder "what did I do wrong? Did I leave something out?" Sixteen, take away eight, is eight. And that is eighteen. The difference between the two is eighteen. If the difference is divisible by nine, as it clearly is, that means you've reversed the number, and in this case, here it is. You reverse the thirty one and the thirteen. Every time you add up columns of information you have to balance out, if the result is divisible by nine, you reverse the number.

Now let's pick a more difficult one. Sometimes were just working with zero one one zero zero one one zero. But sometimes were working with more complicated numbers. Two thousand forty two. One thousand nine hundred and sixty three. And we'll make it easy, eight hundred. This adds up to five, this adds up to ten, that adds up to nine, [ehhh] eighteen I should say, and that would add up to four. Let's say we write it again over here. Two, oh, four, two. Nineteen sixty three, and eight hundred. Seven, eight, seventeen, forty seven, eighty seven. Well they don't seem to match up [hmmm] what's the difference between the two? Fifteen, take-away seven is eight, nine is one, its divisible by nine, its eighteen. So we've reversed the number. Of course I did it deliberately, here it is. Two oh four two, and two oh two four. Any time your balancing out two columns of information, and you need them to be equal, debits and credits for example, if the problem is divisible by nine, you reverse the number.

Now let's look at another way to use the "self-effacing nine." It's called "self-effacing" because you can use it to factor out anything.

This is a fun little game. Let's take another column of information. Sixteen, twenty three, nineteen, forty seven. Let's add this up. I get a hundred and five, is that what you get? Good. [laughs] Now were going to take away everything that adds up to nine. [hmm] Six and three adds up to nine, let's take away nine itself, what else have we got? Seven and two adds up to nine. That leaves us with a one, and a one, and a four, and that equals six. One, plus zero, plus five equals six.

It's one of the interesting things you can do with a nine, it's self-effacing, it can disappear and leave you with a basic bottom line. Sometimes simple arithmetic is the most complicated because we take it for granted and it's easy to make mistakes.

So let's look at one more example and we'll do it with a birthday so you can see how it could be handy in numerology. Let's look at a birthday, may twenty ninth, nineteen fifty two. So May, twenty, nineteen fifty two. That's seven, seven, nine, one, nine, seven, seven. Now I want to check my work and make sure I added this properly. So here's the shortcut with the self-effacing nine. Take away everything that adds up to nine. Nine, five, two, and two, and your left with a five, and a one. Now this adds up to twenty four, equals six. So does five plus one. That means I've checked my work, and it's correct.

The self-effacing nine is handy if you're doing any bookkeeping, columns of information, or if you need to check your work. Remove the nines and it will add up to the same thing in the end."