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# Alternate mental subtraction method

## Video transcript

I want to show you a way that at least I find more useful to subtract numbers in my head and I do it this way not it's not necessarily faster on paper but it allows you to remember what you're doing because if you start borrowing and stuff it becomes very hard to remember what's actually going on so let's try out a couple of problems let's have nine thousand four hundred and fifty six minus seven thousand five hundred and eighty nine so the way I do this in my head I say the nine thousand four hundred and fifty six minus seven thousand five hundred eighty nine you have to remember the two numbers so the first thing I do is I say well it's just what's nine thousand four hundred and fifty six minus just seven thousand right that's pretty easy because I just take nine thousand minus seven thousand so what I can do is I'll cross out this and I'll subtract seven thousand from it and I'm going to get two thousand four hundred and fifty-six so in my head I tell myself that nine thousand four hundred and fifty six minus seven thousand five hundred eighty nine is the same thing as if I just subtract out the seven thousand as two thousand four hundred and fifty-six minus 589 right I took the seven thousand out of the picture I essentially subtracted it from both of these numbers now if I want to do two thousand five for two thousand four and fifty six minus five hundred eighty nine what I do is I subtract five hundred from both of these numbers so if I subtract five hundred from this bottom number this five will go away and if I subtract five hundred from this top number what happens what's two thousand four hundred fifty six minus five hundred or an even easier way to think about it what's 24 minus five well it's nineteen so it's going to be 1919 hundred and fifty-six let me scroll up a little bit so it's 1956 so my original problem has now been reduced to 1956 minus 89 now I can subtract 80 from both that number and that number so if I subtract 80 from this bottom number the eight it disappears right 89 minus 80s just nine and I subtract 80 from this top number I can just think of what's 195 minus eight well 195 mind eight let's see 15 minus 8 is 17 so 195 minus 8 is going to be 187 and then you still have the 6 there so essentially I said 1956 minus 80 is 1876 and now my problem has been reduced to 1876 minus 9 and then we can do that in our head what's 76 minus 9 that's what 67 so our final answer is 1867 and as you can see this isn't necessarily faster than the way we've done it in other videos but the reason why I like it is it at any stage I just have to remember two numbers I have to remember my new top number and my new bottom number my new bottom number is always just some of the leftover digits of the original bottom number so that's how I like to do things in my head now just to make sure that we got the right answer maybe to compare and contrast a little bit let's do it the traditional way nine thousand four hundred and fifty six minus seven thousand five hundred and eighty nine so the standard way of doing it I like to do all my borrowing before I do any of my subtraction so that I can stay in my borrowing mode or you can think of it as regrouping so I look at all of my numbers on top and see are they all larger than the numbers on the bottom and I started here at the right six is definitely not larger than nine so I have to borrow so I'll borrow ten or I'll borrow one from the tens place which ends up being 10 so the six becomes a 16 and then the five becomes a four then I go to the tens place for needs to be larger than eight so let me borrow one from the hundreds place so then that four becomes a 14 right or 14 tens because we're in the tens place and then this four becomes a three now these two columns or places look good but right here I have a three which is less than a five not cool so to borrow again that three becomes a 13 and then that nine becomes an eight and now I'm ready to subtract so you get 16 minus 9 is 7 14 minus 8 is 6 13 minus 5 is 8 8 minus 7 is 1 and lucky for us we got the right answer and now you know I want to make it very clear there's no better way to do this the this way is actually kind of longer and takes up more space on your paper than this way was but this for me is very hard to remember it's very hard for me to keep track of what I borrowed and what another number is and etc etc but here at any point in time I just have to remember two numbers and the two numbers get simpler every step that I go through this process so this is why I think that this is a little bit easier in my head but this might be depending on the context easier on paper but at least here you doesn't have to borrow or regroup well hopefully find that a little bit useful