Fast Math Trick - How to multiply 2 two digit numbers - why it works! - Free Educational videos for Students in K-12

Fast Math Trick - How to multiply 2 two digit numbers - why it works! - By tecmath

Transcript


00:00	good and welcome to Take Math Channel . Why don't
00:03	we have you look at this video ? I'm going
00:04	to be looking at a video that I made about
00:07	a year and a half ago . I'm going to
00:08	be following up just some things with that . What
00:11	we're doing is we're looking at how to multiply two
00:13	digit governments together , get really , really fast results
00:17	. Okay . And that's a hopefully you guys are
00:21	pretty good with this , but there's been a bunch
00:23	of comments where people have been actually asking either how
00:25	this works or they've been getting in these sort of
00:28	discussions . Okay . So I thought I'd make a
00:29	video to do with this . Also you're not going
00:32	to just explain where a couple of people are getting
00:34	in little errors with this as well . Okay ,
00:37	so First off , this is the way the type
00:40	of question we were doing . We're doing this sort
00:42	of one . Um , I've chosen a fairly simple
00:45	115 times 28 . Okay . Damn . What did
00:51	they do before I start on this ? I actually
00:53	just want to quickly jot down the way that you
00:54	previously mind to do this . I'll do this over
00:56	here . Um now 15 times 28 . 15 times
01:02	28 . And the way that you usually do this
01:05	is you're probably used to is used to probably going
01:07	five times eight is 40 . Okay , so I'm
01:10	just going to write this down here . So we
01:11	got the five times eight part and that equals 40
01:16	. Usually carry this across but I'm not going to
01:18	do this this time . I'm going to write at
01:20	all our little parts here . OK then the next
01:22	part we do is 58 and 40 would go to
01:24	this part over here which is uh one times eight
01:28	. Okay . And that one times a part there
01:32	. Usually we carry the four across here but we're
01:34	not doing that this time is Okay but this is
01:37	not one times this is actually 10 times okay so
01:40	we're going to get the answer . Both Heidi .
01:44	Okay . Because 15 the actual one part here is
01:46	not a R one or two tent . Okay .
01:50	So that's that part there . The next thing we
01:52	have is we have usually we draw a little zero
01:55	down here . The way that the reason we do
01:58	that is because this is now we're multiplying This part
02:02	by 20 . Okay so the first ones were multiplied
02:06	by eight . The next parts were multiplied by 20
02:08	. So 2025 is 100 . So this is a
02:14	20 times five part . We usually we see that
02:17	as two times five with zero there . And then
02:21	what we also do is we go with this ,
02:25	This is 10 times 20 . Okay , Which is
02:29	200 . That's a 10 times 20 . 20 times
02:35	10 . And then what would usually do is we
02:37	add all these together ? Okay . 000 04 plus
02:43	eight is 12 carry the 11 plus one is two
02:48	plus two is four . The answer is 420 .
02:51	Yeah , there's a reason I've gone through that big
02:54	arduous long process and the reason for that is this
02:57	monitors try and explain that what we generally do ,
03:00	what we're doing this method about . And do we
03:02	do actually cover all this ? And more applications were
03:05	not getting away with not doing any of that ,
03:07	What we're not getting away with ? Uh you know
03:08	, doing a shortcut in terms of the amount of
03:11	calculations , it just is there faster calculations to do
03:15	? Okay , so I'll give you an example so
03:20	I'll have a look at this . Usually what we
03:21	do this one first office would go One time .
03:24	So again , this is not one times two though
03:26	, is it ? It's 10 times 20 . So
03:28	usually were we do it this method , we're doing
03:31	this one first . So we put it to down
03:33	there , we're going to leave . Usually I'll just
03:35	put it in there . Really , really whatever we
03:37	leave a space . And there's also this bit here
03:39	which I'd say we're going to put the unit .
03:41	So do you realize we're putting down 200 ? I
03:43	just haven't put down the extra zeros there . Okay
03:47	. So we've done the 10 times to the next
03:49	part . We do is this unit's parts is five
03:52	times eight which is 45 times . I'll put a
03:55	little tick there five times a day . Yes .
03:59	40 . Okay . So I'm gonna put the four
04:03	in there right now . The next part we do
04:06	is where we multiply this number by this number .
04:09	The one times eight and the Two times five .
04:12	But it's not actually that at all . It's actually
04:14	10 times eight . Aid . Five times 20 .
04:18	Okay . But we're seeing this we're taking a bit
04:20	of a shortcut here because this is a tense .
04:22	Probably gonna be putting the answer in the tense part
04:24	. So five times two is 10 . 1 times
04:27	eight is 81 times eight is eight . Sorry .
04:30	So there's those two parts . Okay . Eight plus
04:34	10 is 18 . So I'm gonna add it onto
04:36	here . So 18 which is actually 100 and 80
04:39	. I'm adding on here . He's going to Okay
04:42	. So we can put this 18 above here .
04:43	We're going to get the answer of 420 . Okay
04:48	. This is the way this works . Were not
04:50	we still actually have to do every part of the
04:52	multiplication . It's just that this way works a whole
04:55	lot faster . This method over here works faster now
04:59	. Just a couple of things that that's the explanation
05:02	of why it works now . Just one other thing
05:03	that you might think of with this is people also
05:07	occasionally like to put this in a different fashion ,
05:11	which I'll going to different fashion , they like to
05:16	write it like this . Okay , I've seen this
05:18	in the commentary a fair bit to where people have
05:19	it written like this . Okay , but they use
05:22	letters , Okay , I'm not going to use letters
05:23	, I'm going to use these actual particular things .
05:25	So 15 is 10 plus five , 29 is 20
05:30	plus eight . And the way that we get our
05:34	answer is as follows , we're doing the same sorts
05:36	of things . Were starting out , we're going 10
05:38	times 20 is 200 . Okay , So we're putting
05:44	in the two there were leaving a space of space
05:46	because it's 200 . We've done that part there .
05:48	The next what we're doing is the five times eight
05:50	, which is 40 putting it in there . And
05:52	the same thing as we're going 20 times five and
05:55	10 times eight , we're getting 180 , we're adding
05:58	that on and we're getting 400 and 20 . Okay
06:05	. So we're making sure we multiply all these things
06:08	out this way . Just doesn't bother with all these
06:10	little are harder looking parts down here . Not that
06:13	they are harder . They're exactly the same . Okay
06:17	, now , just a couple of things that people
06:18	seemed . Thank you for any comments to do with
06:21	that . It's been really good that people have got
06:24	on there and then put comments and that sort of
06:26	deal and try to explain it because because , you
06:29	know , it really helps everybody out . I think
06:31	now just one little thing , a few people are
06:34	getting stuck on various ones and the reason for this
06:37	generally , which I see is It's to do with
06:41	carrying . I'll give you an example of this and
06:43	I'm going to go a bit of a funnier one
06:46	now I'll go a bit of a one . Okay
06:47	, let's go something like say 73 times 24 .
06:54	Okay , so Let's start out with this seven teus
07:00	of 14 . Okay , so we put the 14
07:02	down , we put a space . Okay , because
07:05	the 14 as you might realize is actually 70 times
07:09	20 , which is 1400 . So I know there's
07:12	another space here . We're going to fill that in
07:13	with our units in a second , Which is this
07:15	one , three fours , which is 12 . So
07:19	I'm gonna put the two here . I might even
07:20	put the one up here for a minute because I
07:22	want to see how we go with this one .
07:24	I don't know , I haven't actually done this question
07:26	yet , so we'll see how we go . Uh
07:28	74 28 28 plus three times two , which is
07:34	six is 34 So plus one is 35 . So
07:39	we're gonna put the five there . Then we have
07:42	to carry this story across this by the way .
07:46	Yes , 70 times four . I'm gonna write that
07:49	down 70 times four , which is 2 80 head
07:55	Through three times 20 . Okay , so 20 times
08:00	three Which is 60 and this is actually um And
08:05	we're gonna add a 10 into this as well because
08:07	that's that part were carried over here . So what
08:09	we're getting is we actually have 350 . Do realize
08:12	I'm not putting 35 in here , I'm putting 350
08:15	, but I only have one space in here .
08:17	So I have to carry this part across to the
08:20	hundreds . Okay , so the fires in there ,
08:23	this is zero , so it's not going to actually
08:25	matter . And the three over here is going to
08:27	four plus three is seven . So the answer Is
08:35	1,752 . So if you're getting your mistakes with these
08:38	, trying to make sure that you are carrying things
08:40	across as you should be okay . And thanks to
08:43	people who have answered other people's questions regarding this ,
08:47	I've tried to get on the odd time myself ,
08:49	but I must say , you know , I also
08:51	get a lot of busy with other things as well
08:53	. So thanks for people for doing that . Anyway
08:55	, I'm gonna put a link up to that previous
08:57	video . Thanks for watching . See you next time
00:0-1	.

Summarizer

DESCRIPTION:

OVERVIEW:

Fast Math Trick - How to multiply 2 two digit numbers - why it works! is a free educational video by tecmath.

This page not only allows students and teachers view Fast Math Trick - How to multiply 2 two digit numbers - why it works! videos but also find engaging Sample Questions, Apps, Pins, Worksheets, Books related to the following topics.