Fast Math Trick - How to multiply numbers under twenty faster than a calculator! - Free Educational videos for Students in K-12

Fast Math Trick - How to multiply numbers under twenty faster than a calculator! - By tecmath

Transcript


00:0-1	Hey and welcome to the tech Math channel today ,
00:03	we're gonna have a look at a special little tricky
00:04	mats and this is how to multiply two numbers together
00:08	. Really , really quickly numbers that are a little
00:10	bit beyond your standard times tables , questions the numbers
00:12	that start to get into the teens , things like
00:14	13 times 15 and 11 times 19 and things like
00:18	this . So you should be able to do these
00:22	really , really quickly after I'm finished with you today
00:24	and what you've done a little bit of practice and
00:25	won't take that long . It's just a couple little
00:27	tricks and once you do them , it's a little
00:29	secret that makes you look really , really smart .
00:31	A little secret that only we will know about you
00:33	, me and rest of Youtube . So Well ,
00:37	see there you go . So first off , I'm
00:39	gonna give you a little question to start yourself ,
00:41	get your brain working . So the question I'm going
00:43	to give you , see if you can witness out
00:44	in four seconds is 12 times 14 . Ready set
00:48	go . Hey , did you get the answer of
00:51	168 ? If you did well done , you probably
00:54	are almost gonna be good enough necessary . You know
00:57	, probably you might not need to listen to me
00:58	anymore , but unless you have 100% sure and you
01:02	reckon you can work out even things like 17 times
01:04	19 and things like this that quickly , maybe you
01:07	want to stick around . So if you didn't ,
01:09	don't worry about this is what we're going to do
01:11	it right now , we're going to have a bit
01:12	of a look at this and what you'll be able
01:14	to do is soon you'll actually be able to do
01:15	it faster than a person can actually plug the numbers
01:18	into a calculator . Alright . It's always pretty impressive
01:20	when you can do this . So uh this always
01:23	spends my students out in class and you know they
01:26	go to work something out , they put a calculator
01:28	to work it out and then when you can answer
01:29	it really quickly they think you're a real genius ,
01:31	but it's not real genius , it's just a little
01:33	treat . Okay , so let's have a look at
01:36	this right now . So the way that you guys
01:37	are used to doing it , the either politic calculator
01:39	, Hey , but we all know that's that's sort
01:40	of cheating yourself . So let's do it the old
01:42	way that you do on paper . So 12 times
01:45	14 , you probably do it like this 12 times
01:48	14 . And what you got taught in school is
01:50	you go two times four , you get 81 times
01:53	four is four and then you get to the 10th
01:56	until you go put the 10 there , and then
01:58	one times two is two and one times one is
02:01	one . And then you have to add these numbers
02:03	together and you get eight and you get four plus
02:06	two or six and then you get a one there
02:08	and hopefully I haven't run out of space . They're
02:10	just about having really , where that really really well
02:12	. Um but you're never gonna get that done in
02:15	four seconds like that no matter I think how far
02:17	she tried , I think you'll be able to write
02:18	it that fast , let alone working out in your
02:20	head . So let's get rid of that . I'll
02:23	show you the way that a faster way of doing
02:25	a smarter way of doing it . Okay , A
02:27	little trick . So First off the way this works
02:30	is the major thing , you have to know where
02:32	this is , how far the number is from ,
02:33	the number 10 . So first off let's just drop
02:35	this down but Let's have a look at this .
02:38	So the number 12 , how far is this from
02:40	the number 10 ? That's too the number four day
02:43	. And how far is that from ? The number
02:45	10 ? Well it's four . Well first off we're
02:48	going to work out the tens number . There's two
02:49	things you have to work out of the tens number
02:51	and the units number . Okay , so first off
02:54	the tens number . Get this number here , 12
02:59	and add it to the four . Okay , so
03:01	we're gonna add these two numbers together . 12 plus
03:03	four . And this is going to give us the
03:04	tens number 12 plus four is 16 . What you
03:07	might notice when we're starting out the two plus 14
03:09	gives us exactly the same answer . So it actually
03:11	doesn't matter As long as you sort of add these
03:13	across diagonally , you get this 116 because it's a
03:17	tens number . It's actually 160 something . So I
03:20	leave a little blank there . I think you have
03:21	to work at the units number . And the way
03:23	you do this is you , are you multiply these
03:25	two numbers together . Two times four . The answer
03:29	is 168 . That's pretty quick . Right ? Hey
03:34	, I'll show you are going to get how to
03:35	do this and I reckon you you'll get really ,
03:37	really fast at doing it . So what about I'll
03:39	give you another question here . What about we give
03:42	you something ? Uh Say we give you the question
03:44	. Um What about 13 times 12 ? So once
03:51	again , this is three from 10 . This is
03:53	two . More than 10 . So three Plus 12
03:57	is 15 . It's a tense number . Then two
04:01	times 3 is 656 . Cool . Right . What
04:07	about to give you another one ? I'll get rid
04:12	of that . What about the next one ? Will
04:14	do something like a bit harder this time , 18
04:19	times 14 . So This is eight more than 10
04:24	. This is four . More than 10 . Mhm
04:28	. Okay . Yeah . eight plus 14 . We're
04:32	going to get the answer at 22 So to 20
04:35	and that's our tens number once again . But when
04:37	you can see this time is when we multiply four
04:39	and eight together , we get the answer of 32
04:41	. Now , this is a two digit number .
04:43	You wait a second . How do you put a
04:44	two digit number on in one space here ? That's
04:46	how it makes it a little bit harder . Well
04:49	, first off put the units number down there ,
04:51	so the two and with the three , we just
04:53	carry that across . So we get the answer .
04:57	203 Plus two is 5 two . So 2 52
05:03	. That's our answer . Pretty simple . Yeah .
05:06	Hey look I was going to try and get this
05:08	done within five minutes but I reckon I was going
05:10	to go one more . If you're really really certain
05:12	of what you're doing there , you're thinking I've had
05:14	enough of this . I've worked it out . That's
05:15	really good . And you're probably wanting to practice in
05:17	yourself so you might disappear at this minute for you
05:19	guys who don't I'm gonna actually are be a little
05:22	bit nicer , a bit slower on this one and
05:24	I'm gonna give you one last question and this will
05:26	be say let's give it one last one . A
05:28	really hard 1 17 times 19 . So your turn
05:34	. What's the first thing we do again ? Okay
05:36	. We work out how much above 10 it is
05:38	so We've got seven above 10 and nine above 10
05:43	. Next yeah , we had these two guys together
05:46	, so seven plus 19 is 26-60 something Then 7963
05:54	. Okay , 63 . So we have these together
05:57	, we get 300 And 23 is our answer .
06:03	Really good . I'll give you a little hint with
06:05	this . Now I'm showing you this one way of
06:09	doing it . Actually do it a little bit differently
06:10	and oh my God , you're saying what ? He's
06:12	just showing us one way . Why did you bother
06:14	? And then you turn it all around just when
06:15	you're trying to work this out in your head ,
06:16	because you hit this stage , you start to be
06:18	able to think , well hang on a sec ,
06:19	do I even need to write it down as much
06:21	when I'm doing these ? Actually do it a little
06:23	bit differently . Um Now , once I once I
06:26	got used to these , I started doing it a
06:28	little bit differently . I no longer write down the
06:29	seven and nine because they're already written here . I
06:32	don't need to write them down twice . What actually
06:34	you can do here is I actually , first off
06:37	I go to seven times nine and I keep that
06:38	in my head . So 79 a 63 and then
06:41	63 63 keep that in your head . And seven
06:45	Plus 19 is 26 , 260 plus , 63 Is
06:50	, three , Okay practices enough . You'll be able
06:55	to do them really , really fast , Okay ?
06:57	And you hit this stage , you can do them
06:58	really , really quick , right a couple down ,
07:00	give it a practice to get used to doing them
07:03	. I'll tell you what will happen . You go
07:04	after your math class or whatever like this , you'll
07:06	have your teacher , they'll go , what's this year
07:08	are They'll go to pull out a calculator and you'll
07:10	be able to do in your head faster than your
07:12	teacher can calculate it . They will put the wind
07:15	right up and right anyway , If you have any
07:18	questions on how to do that , if you're not
07:20	100% sure you asked me a question and I'll answer
07:24	that for you anyway . I hope you're happy with
07:26	that . See you next time .

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Fast Math Trick - How to multiply numbers under twenty faster than a calculator! is a free educational video by tecmath.

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