0:00:00.467,0:00:03.509
I want to show you a way that,[br]at least, I find more useful to

0:00:04.428,0:00:05.346
subtract numbers in my head.

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And I do it this way-- it's[br]not necessarily faster on

0:00:07.931,0:00:10.668
paper, but it allows you to[br]remember what you're doing.

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Because if you start borrowing[br]and stuff it becomes very hard

0:00:12.724,0:00:14.603
to remember what's[br]actually going on.

0:00:14.603,0:00:16.417
So let's try out a[br]couple of problems.

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Let's have nine thousand four hundred fifty-six minus seven thousand five hundred eighty-nine.

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So the way I do[br]this in my head.

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I say that nine thousand four hundred fifty-six minus[br]seven thousand five hundred eighty-nine-- you have to

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remember the two numbers.

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So the first thing I do is[br]I say, well, what's nine thousand four hundred fifty-six

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minus just seven thousand?

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That's pretty easy because I[br]just take nine thousand minus seven thousand.

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So what I can do is I'll[br]cross out this and I'll

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subtract seven thousand from it.

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And I'm going to get two thousand four hundred fifty six.

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So in my head I tell myself[br]that nine thousand four hundred fifty-six minus seven thousand five hundred eighty-nine is the

0:01:05.414,0:01:08.667
same thing as-- if I just[br]subtract out the seven thousand--

0:01:08.667,0:01:12.966
as two thousand four hundred fifty-six minus five hundred eighty-nine.

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I took the seven thousand out[br]of the picture.

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I essentially subtracted it[br]from both of these numbers.

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Now, if I want to do two thousand four hundred fifty-six[br]minus five hundred eighty-nine what I do is I

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subtract five hundred from both[br]of these numbers.

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So if I subtract five hundred from[br]this bottom number,

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this five will go away.

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And if I subtract five hundred from this[br]top number, what happens?

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What's two thousand four hundred fifty-six minus five hundred?

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Or an easier way to[br]think about it?

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What's twenty-four minus five?

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Well, that's nineteen.

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So it's going to be one thousand nine hundred fifty-six.

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Let me scroll up a little bit.

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So it's one thousand nine hundred fifty six.

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So my original problem has now[br]been reduced to one thousand nine hundred fifty-six minus eighty-nine.

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Now I can subtract eighty from both[br]that number and that number.

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So if I subtract eighty from this[br]bottom number the eight disappears.

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Eighty-nine minus eighty is just nine.

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And I subtract eighty from this top[br]number, I can just think of,

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well, what's one hundred ninety-five minus eight?

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Well, one hundred ninety-five minus eight, let's see.

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Fifteen minus eight is seventeen.

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So one hundred ninety-five minus eight is going[br]to be one hundred eighty-seven and then you

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still have the six there.

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So essentially I said,[br]one thousand nine hundred fifty-six minus eighty is one thousand eight hundred seventy-six.

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And now my problem has been[br]reduced to one thousand eight hundred seventy-six minus nine.

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And then we can do[br]that in our head.

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What's seventy-six minus nine?

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That's what?

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Sixty-seven.

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So our final answer is one thousand eight hundred sixty-seven.

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And as you can see this isn't[br]necessarily faster than the way

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we've done it in other videos.

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But the reason why I like it[br]is that at any stage, I just

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have to remember two numbers.

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I have to remember my[br]new top number and my

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new bottom number.

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My new bottom number is always[br]just some of the leftover

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digits of the original[br]bottom number.

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So that's how I like to[br]do things in my head.

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Now, just to make sure that we[br]got the right answer and maybe

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to compare and contrast[br]a little bit.

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Let's do it the[br]traditional way.

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Nine thousand four hundred fifty-six minus seven thousand five hundred eighty-nine.

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So the standard way of doing[br]it, I like to do all my

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borrowing before I do any of my[br]subtraction so that I can stay

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in my borrowing mode, or you[br]can think of it as regrouping.

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So I look at all of my numbers[br]on top and see, are they all

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larger than the numbers[br]on the bottom?

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And I start here at the right.

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Six is definitely not larger[br]than nine, so I have to borrow.

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So I'll borrow ten or I'll[br]borrow one from the tens place,

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which ends up being ten.

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So the six becomes a sixteen and[br]then the five becomes a four.

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Then I go to the tens place.

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Four needs to be larger than[br]eight, so let me borrow one

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from the hundreds place.

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So then that four becomes a fourteen[br]or fourteen tens because

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we're in the tens place.

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And then this four becomes a three.

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Now these two columns or places[br]look good, but right here I

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have a three, which is[br]less than a five.

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Not cool, so I have[br]to borrow again.

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That three becomes a thirteen and[br]then that nine becomes an eight.

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And now I'm ready to subtract.

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So you get sixteen minus nine is seven.

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Fourteen minus eight is six.

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Thirteen minus five is eight.

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Eight minus seven is one.

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And lucky for us, we[br]got the right answer.

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I want to make it very clear.

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There's no better[br]way to do this.

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This way is actually kind of[br]longer and it takes up more

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space on your paper than this[br]way was, but this for me,

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is very hard to remember.

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It's very hard for me to keep[br]track of what I borrowed and

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what the other number[br]is and et cetera.

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But here, at any point[br]in time, I just have to

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remember two numbers.

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And the two numbers get[br]simpler every step that I

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go through this process.

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So this is why I think[br]that this is a little

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bit easier in my head.

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But this might be, depending on[br]the context, easier on paper.

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But at least here you didn't[br]have to borrow or regroup.

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Well, hopefully you find[br]that a little bit useful.