Subtaction of numbers mentally - Fast Math Trick - Free Educational videos for Students in K-12

Subtaction of numbers mentally - Fast Math Trick - By tecmath

Transcript


00:0-1	get a welcome to Tech Math channel . What we're
00:02	gonna be having a look at this video is we're
00:03	going to be having a look at how to mentally
00:05	subtract numbers from one another . These are going to
00:09	be bigger numbers . We had a look in the
00:10	last video . It's subtracting just two digit numbers from
00:13	one another and how we can do this mentally left
00:15	or right . And we're gonna be using similar techniques
00:18	in working out how to do this with bigger numbers
00:21	now and there is a couple of little extra parts
00:25	on this and it really worth knowing . So if
00:27	you haven't seen that last video , I really recommend
00:28	you go have a look but you could probably get
00:31	by without knowing it as well . So , you
00:33	know , you might be okay . So there's a
00:36	couple of different techniques . We're gonna be using this
00:38	depending on the type of subtraction question we can .
00:41	So I'm going to start out fairly simply . So
00:46	you were asked to take law tried into an example
00:50	. The first type of Mental subtraction that we're gonna
00:52	be looking at is where you get a question like
00:54	this . So you get 583 and you're asked to
00:58	take away 200 and 71 . Okay , Now this
01:05	is a fairly easy question . Remember that we're trying
01:07	to do this mentally as well , so don't just
01:11	start going , okay , well this take away ,
01:13	this is how you would do it . Are how
01:16	you do it written down . See if you can
01:17	start trying to do this mentally . Oh , do
01:19	be a writer here to try and explain things ,
01:21	but try to do this mentally as much as you
01:24	can . Okay , So you're probably gonna realize with
01:27	this 271 is made up of three parts , is
01:30	made up of 200 , It's made up of 70
01:34	and it's made up of what ? So we're gonna
01:37	take all of these off . 583 . There's super
01:40	nice and easy because there's no carrying across required .
01:44	Okay , So this is our first type of question
01:46	, a diocese he won't know carrying or anything like
01:49	that . Okay , so 533 and I'm gonna take
01:52	off 200 and I'm gonna take off 70 and I'm
01:54	gonna take off one . I'm gonna try and do
01:55	this all mentally . So this is what you want
01:59	to be hard to practice with these . 583 ,
02:03	383 . 313 . 312 . Okay . Up to
02:14	300 ad . That's the way we're writing to use
02:17	these days . We've decided to ride them right there
02:19	. So it's 312 . Okay , So you see
02:25	with this , I've only read this part up here
02:27	for your convenience , but we typically don't need to
02:30	do this . And I think a few people are
02:34	going to be fairly okay doing these straight away So
02:37	I'll give you another example of this . So you
02:39	had 936 and we're going to take away 725 .
02:49	So how are we going to do this ? Okay
02:53	. 936 . 236 . 216 . 200 and he
03:04	left . Okay , Just see what I did is
03:10	I went 936 takes 720 take five . It's like
03:16	I so do you get the hang of those ?
03:21	Okay . What about I'm going to give you uh
03:25	The next type of example that we've been looking at
03:27	because they're fairly easy . I think they're fairly basic
03:29	to understand . But say you start to get questions
03:31	that are a little bit different . So you start
03:33	to get say something like this where we have 687
03:38	and we want to take away . And I'm gonna
03:41	start out fairly basically these as well 298 . They're
03:46	going to see what this is , not a number
03:48	where we're gonna have to actually borrow here . But
03:51	What you notice is that 298 we're going to be
03:54	looking at this number first off . That's fairly close
03:58	to 300 . Okay so we're gonna round up This
04:00	double of 300 and to do this you're going to
04:05	see that we've added to . Okay so I was
04:07	going to right this as well . So this is
04:10	added to Just to remember these for the minute .
04:13	You won't need to do this as much all this
04:15	writing when you're first when you're doing things a bit
04:18	later on . So what we're gonna do with this
04:20	is gonna take off 300 from 687 , but because
04:24	we're gonna Take two too many , because 298 is
04:27	too Less than 300 , we're going to add that
04:31	to back on afterwards Because the 687 Take away ,
04:35	300 and 387 Add that on to get 300 at
04:41	80 . Doing okay . All right . Uh So
04:50	it's a nice easy way of exemplifying how we do
04:53	that . So we're going to be taking too much
04:56	off or occasionally taking a little bit often they're taking
04:59	a bit more off and I'll show you how we're
05:00	gonna do this . So I'm going to go through
05:01	a bunch of examples we'll deal with as we go
05:04	. So say you had 304 and we wanted to
05:09	take off 185 . Okay . So hopefully what you're
05:19	going to see is this is getting closer to 200
05:24	. But to get 200 you might realize that we
05:27	need to We're taking off 52 burning . Okay .
05:31	So that Put a plus 50 there . So when
05:34	we do this , so I'm going to get to
05:35	this how to work these plus 15 to that out
05:38	because at the moment we're sticking numbers fairly close to
05:40	100 . I've done this on purpose , but I
05:42	think in a second I'll show you how to find
05:44	these numbers out fairly easy and then we'll get back
05:46	to these again . So 300 and four take away
05:49	300 is 104 Plus 15 is 119 . Okay .
05:58	Yeah , they're fairly basic , but I just want
06:01	to get to this stage where so you were given
06:04	something where we wanted to Take off a number and
06:08	the number was 346 . We're going to see straight
06:12	away . This is not close to 100 . And
06:14	some people do struggle working at what number would need
06:17	to to get up to 400 . I'll show you
06:19	how to do this . The way we do this
06:22	is through this idea of cops , eventually numbers .
06:25	Uh and I'll give you an example these say 46
06:30	. It's a complimentary number is a W would add
06:32	to a 46 to get 100 . Okay . And
06:37	the really , really easiest way of doing these .
06:40	Think of what number you would add to four to
06:42	get died . And then what number you would add
06:44	to this units one to get ? Okay , So
06:50	four plus what equals nine ? It's five . Six
06:55	plus what equals 10 is four complementary to 46 is
06:59	54 . So 50 forward where the double we're adding
07:01	on . Okay . What about a complimentary double for
07:05	uh say the number was 73 . We're going to
07:11	see straight away . Seven plus two equals 93 plus
07:15	seven equals stand in . A complementary number is 27
07:19	. A lot of things to remember . Is there
07:21	? What about this ? What about ? We had
07:24	68 . Well you're gonna start seeing these quicker and
07:28	quicker . This is going to be 30 to 6
07:31	plus three equals 98 plus two equals 10 . So
07:34	these are complementary numbers . We'll get back to our
07:36	questions and now that , you know , But as
07:38	complementary numbers actually let me just shot down a couple
07:41	of these . So you have the number 84 19
07:50	, So what are the complementary doubles to these ?
07:52	You're gonna see this one is 16 . This one
07:56	is going to be 81 . This one is going
08:00	to be 73 again . This double that up tonight
08:03	. Is that up to 10 ? Easy . Okay
08:09	, let's do a few more the subtraction questions to
08:11	say we had 219 . And we're going to take
08:17	away . Actually . That's a it's not a really
08:20	great number . We're gonna we're gonna take About 455
08:27	and we will take away 319 . Okay , So
08:34	How are we going to do this to complementary to
08:36	319 . 1st off , this is going to go
08:38	up to 400 and complementary to that is going to
08:41	be what ? It's going to be 81 ? Yes
08:44	. Okay , so let's do this . Uh this
08:47	is going to become a 400 . Okay , so
08:51	400 take away 455 . Take away . 455 ,
08:56	55 plus 81 Is 130 six . Okay , another
09:08	example here , what about we do 16 82 .
09:14	Well , there's a 680 to take away 385 .
09:20	And so you're going to see the complementary w this
09:22	is 15 , this is going to become a 400
09:25	. Well , there's a 680 - 1680 to take
09:28	away . 400 is Well , there's 282 Plus 15
09:34	is going to be 1297 . Okay , now there's
09:42	a couple of slight variances , you're gonna start varying
09:44	these a little bit , depending upon the question .
09:46	What makes more sense to do ? So say you're
09:48	giving something a little bit different , say it was
09:51	693 , and I say take away 302 from this
09:59	. Well , the easiest way with this might straightaway
10:01	be actually this is just going to be using the
10:03	first method . Okay , but say I did not
10:07	make this a bit bigger , everybody is 307 .
10:11	Okay . I think the easiest way to be ,
10:13	because I've been around this actually the 300 , but
10:15	then I'm going to take away , more . So
10:19	first I'm going to take away 300 and then I'm
10:21	going to take away seven board . So this is
10:23	going to become 393 , 393 . Take away seven
10:30	is going to be 380 six . Okay , so
10:37	there is some slight variances with these . Uh Anyway
10:44	, hopefully you're finding these not too bad to start
10:46	writing down a few of them by yourself and practicing
10:48	these . It's it's a really great little method .
10:51	You should be able to start getting these really ,
10:54	really rapidly . I'm just heading this stage , I
10:57	was writing it down randomly and while I'm talking and
11:00	you should be able to start even working these out
11:02	. So What's the answer to this 1 ? Give
11:05	it a couple of seconds , see if you could
11:07	work it out , Are you going to be taking
11:09	off 600 , this is going to be adding 29
11:12	to it . So we're going to end up with
11:13	14 128 take away 600 is 828 , 828 .
11:23	800 48 by 800 and 57 . Anyway , hopefully
11:34	you find this method handy . If not , don't
11:36	use it . Okay , anyway , see you next
11:38	time . Bye . Yeah .

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