Mental Math Tricks - How to multiply in your head - Free Educational videos for Students in K-12

Mental Math Tricks - How to multiply in your head - By tecmath

Transcript


00:00	Good day and welcome to the Tech Math channel .
00:02	Why don't we have to look at in this video
00:03	is a very special video . We're going to be
00:05	looking at how to multiply mentally how to multiply big
00:09	numbers mentally . So bear with us , you might
00:12	look at the party and say , hey , I
00:14	can do this much faster . The pen and paper
00:16	. But this isn't about trying to multiply with a
00:18	pen and paper . This is actually trying to get
00:20	you eventually multiplying mentally , which is where the real
00:24	our challenge lies a lot of the time . Okay
00:27	, so I'm going to start this off with an
00:29	example , let's get you doing this one . What
00:32	is a nice easy one to start off with 3,021
00:36	and we're going to times that by three . So
00:38	the answer that you should get for this is 9000
00:43	And 63 . Did you get that answer ? You
00:47	know what I reckon you probably did and I'll show
00:49	you how I reckon you probably got that , How
00:51	I think you probably got that . Okay . I
00:54	would expect the way you've done that as you would
00:57	do this because you're doing this a written method ,
00:59	you would go one times three is 32 times three
01:02	is 63 times nothing is nothing , three times three
01:06	is nine . Okay . Yeah . Look that's nice
01:10	and easy if you're doing it on paper , but
01:13	it does become a little bit harder all of a
01:15	sudden when you're not able to actually see the numbers
01:18	. Okay . And in a second I'm going to
01:20	actually I'll give you one without the numbers , then
01:23	I'll actually right at the animal work through how your
01:25	mind might want to get reprogrammed because I do this
01:27	slightly differently . Instead , what we tend to do
01:30	is we tend to work out questions this way from
01:34	right to left , whereas what we actually , when
01:37	we think of numbers , we actually think about them
01:39	this way , okay , we don't think about this
01:41	number being three and 60 and 9000 . We think
01:44	about this is 9000 and 63 . And this is
01:47	the challenge with multiplying mentally , is to turn the
01:50	way around that you currently work your multiplication . So
01:55	you think about them in the way that you actually
01:57	think about them , okay , in that particular way
01:59	, rather than actually working against the way your mind
02:02	naturally wants to work . Okay ? So the way
02:05	that I would do this is a little bit different
02:07	and this is what I mean with how we have
02:10	to change the way that we are Work , we
02:13	do this , we're times is by three , but
02:15	instead what we would do is we would go three
02:18	times three is nine 03 times two is 63 Okay
02:24	. And we're actually working this in the way that
02:27	we actually you know that you think the number ?
02:30	Okay , I'll give you another example so I'll rub
02:34	this out and then I'm going to ask you For
02:38	example to work out what is 51,231 times three .
02:44	Okay . So it's quite hard if you are if
02:48	you're not used to doing in that direction . So
02:52	the answer by the way is 100 and 53,693 .
02:58	Okay let's uh I'll write that down so let's I'm
03:01	gonna write the question down first . Um 51231 .
03:08	It was Times three . So the way we do
03:12	it is this Three times five is 15 . We
03:15	put that down , so we're going from left to
03:17	right , five times 3 is 15 , three times
03:21	1 is three , two times three is 63 times
03:26	three is 91 times three is three . You see
03:31	we're working this way and this is the trick to
03:34	mental calculations . Mental multiplication . So I know what's
03:38	going to happen . I'm going to start getting probably
03:40	comments people saying this is harder , this is not
03:42	what I'm used to because it's not what you're used
03:43	to . But then again you also might realize that
03:47	the method you used to is no good for mental
03:49	multiplication . So this is it does take a bit
03:51	of practice , but this is the way to do
03:53	it mentally . This is well one of the ways
03:56	to do it mentally , so I'm going to give
04:00	you a another one now . What about we deal
04:03	with ? Hopefully you do pretty good with this .
04:06	Um What about we do a different one here .
04:09	What about we do something ? Say like a 61413
04:14	. I'll write that number down , 61413 and all
04:20	times that number by four now from now on .
04:23	Rather than saying them . I'm going to actually you
04:25	know so without actually writing down you might go four
04:28	times six is 24 . 1 times four is for
04:31	then The 16th . Wait see hanging to deal with
04:33	this carry here . What you say ? I don't
04:36	even know what just went on . I will explain
04:38	to you slowly what the problem will be here .
04:40	Okay so what we're doing this we have a bit
04:42	of a problem because what we do is we go
04:44	six times four is 24 one times four is four
04:49	. Four times four is 16 . Wait a second
04:52	. That means we're gonna have to carry something so
04:53	we're gonna put the six there , We're going to
04:56	carry the one . So this is going to become
04:57	a five one times four is four and then 34
05:03	12 . So the two is going to go there
05:05	and the ones going to get carried across . And
05:07	that will also become a five . Yeah , the
05:12	carrying all of a sudden can become a bit of
05:14	a problem when we talk about mental calculation , because
05:17	carrying numbers in your head tends to throw things a
05:19	little bit . What we tend to have to do
05:21	is we have to carry as we go . But
05:23	we also have to start doing this little thing which
05:26	I call pre carrying . Okay ? It just involves
05:28	glancing at the next part of your answer to to
05:30	work out what what your your current answer will be
05:33	. I'll show you what I mean by this .
05:35	Okay ? And then we'll go through a couple of
05:37	examples with this . So say for example I do
05:41	ah this one here 31,616 times this number by four
05:50	. Okay . I work through how this concept of
05:55	pre carrying works . So first off we do this
05:58	three times four and we get the answer at 12
06:00	. But before right down 12 I look at the
06:02	next part of the answer and I go one times
06:04	four the answers below 10 . I'm not going to
06:07	have to carry anything So I can put down my
06:10	12 . If I did have to carry something I'd
06:12	carried across and put that as part of my answer
06:14	and you'll see how I mean on this one right
06:16	now , one times 4 is four . So I
06:19	quickly look at the next part and 64-24 24 .
06:23	I'm going to have to carry this too . Okay
06:26	so this is what I'm going to pre carry on
06:28	a pre carried across . So one times four is
06:31	four plus that too from the next part is going
06:33	to be six . Okay . All right we'll see
06:37	how we go with that . Um so the next
06:40	one was six times four is 24 . We've already
06:42	written down that too . They were pre carried that
06:44	the number afterwards is one times four we're not gonna
06:46	have to carry anything across so I can just put
06:48	down at four . Okay For at one times 4
06:55	I look at the next part . Okay and that's
06:58	six times four is 24 I'm gonna have to pre
07:00	carry to across . Okay so one times four is
07:02	four plus two is six . Okay . Did you
07:07	get that ? And then the last part ? Well
07:09	that's just going to be whatever that units part was
07:12	going to be 6424 . So that's where that four
07:15	goes down . But this is the concept of pre
07:17	carrying . Okay . And it's a it's a the
07:20	biggest part of this trick is to be able to
07:21	get the pre carry right ? Okay , we can
07:23	get the pre carry right , You're really on the
07:26	way to getting these mentally are correct . So what
07:30	about I give you another example ? Okay , uh
07:35	we'll write this one down . What about for instance
07:41	21,000 613 and so first off , what we do
07:54	going from left to right two times 5 is 10
07:59	. Now we look at one times five no pre
08:01	carry so I can put down my tent . Okay
08:04	? I'll put a little tick there to show that's
08:06	where I've done it . Okay ? one times 5
08:09	. Okay That's going to be five and I'm gonna
08:11	have to pre carry six times 5 30 . I
08:14	have to pre carry three . Okay ? With six
08:16	times five is 30 I had to pre carry three
08:18	so one times five is five plus three . Okay
08:23	is going to be okay ? All right I'll take
08:28	that one off . Six times five is 30 .
08:31	I've already pre carried the three so I've got that
08:33	zero and then one times five . I'm not gonna
08:35	have to pre carry anything so With that I can
08:39	put down zero From the 30 . Think I've already
08:42	pre carried the three that one down one times five
08:46	is five . And then I look at the next
08:48	13 times five is 59 . At the pre carry
08:51	. You cast her one ? Okay because 35 to
08:54	15 I got to pre carry the one off the
08:56	15 so five plus one his six . Okay .
09:02	And then what I do is I put down that
09:05	last bit which was that five ? How did you
09:09	go with that ? Did you get that answer ?
09:10	Did you get 108,065 . Hopefully you'll be able to
09:16	actually are start seeing how to do this . It's
09:19	all a matter of working out the pre carry .
09:22	Okay ? Is it easy at the start ? No
09:26	this does require practice . And what I'd recommend is
09:28	you would start with smaller numbers at the start off
09:31	with . Okay you would start with say something uh
09:34	like something like uh possibly you know What about 423
09:42	and we're gonna times that by four . Okay so
09:48	that's a good one because it's a nice three digit
09:50	, It's not too huge . Four times four is
09:53	16 , 2 times four is eight , so it's
09:55	less intense . I can put down the 16 ,
09:56	I can think 16 . I can keep that in
09:58	my head . Two times four is 83 times four
10:01	is 12 . I have to pre carry that one
10:03	. So it's gonna be a nine and two .
10:07	Did you go with that ? Hopefully . Pretty good
10:10	. Anyway , see here you go with that .
10:12	Before you start getting upset and saying you despise this
10:14	method or whatever like that . Give it a fair
10:17	go because this is trying to reprogram your mind and
10:20	so you are you will actually be able to do
10:22	this mentally . Okay But don't expect that it's going
10:24	to come to you in three seconds flat . Okay
10:28	this does take a little bit of practice sometimes .
10:30	This does take a little bit of practice , these
10:32	sorts of things anyway . With a bit of practice
10:35	, your mental math will get faster . Good luck
10:37	on that anyway . Thanks . See you .

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Mental Math Tricks - How to multiply in your head is a free educational video by tecmath.

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