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Dressed to the Nines

  Posted by RC Lewis , 13 February 2012 · 72 views

We have a wonderful base-10 number system. It makes a lot of things easy, and would make things even easier if the U.S. would get with it and switch to the metric system. Think about the poor Romans. Have you seen those years noted at the end of movies made in the twentieth century? Yuck.

The nature of the base-10 system makes for some interesting things with the number that's just one shy of ten—nine. While learning your times tables, you may have noticed these properties of nine.

Up to 9 × 10, the digits of the products add to nine.

Again up to 10, there's a cool bookend-reversing thing going on, as the first digits go up and the second digits go down:

09
18
27
36
45
54
63
72
81
90

A side-effect of this, along with the fact we have ten fingers, is a little trick I use with kids who still struggle to remember multiplication facts with nine. (I thought everyone knew this, but have found several adults who've never seen it, so I figured I'd share it here.)

Hold your hands in front of you, fingers spread. Whatever number you want to multiply nine by, count that many fingers from the left and put down the finger you land on. (So if you're doing 3 × 9, count three fingers from the left, and put down your left middle finger.)

How many fingers are up to the left of the lowered finger? (In the example, two.) How many fingers are up to the right? (Seven.) Put those together, and you have the answer. (Two and seven ... 3 × 9 = 27.)

And now, I have to go do some research on the title of this post, because I've always kind of wondered about that phrase.


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