Math tricks for fast calculation - Free Educational videos for Students in K-12

Math tricks for fast calculation - By tecmath

Transcript


00:01	good . I welcome to Tech Math Channel . What
00:03	we're gonna be having a look at in this video
00:05	is a really fast instant method . Pretty much of
00:08	multiplying by some pretty common numbers . The numbers we're
00:11	gonna be having a look at Is when you're multiplying
00:14	a number by five . Or if you're multiplying by
00:16	number by 25 or even a number By 125 and
00:21	how you can work these out instantly in your head
00:25	. So let's have a look at this . I'm
00:26	going to start out by looking at five . Then
00:28	we're gonna have a look at 25 then we'll look
00:29	at 100 and 25 were also going to look at
00:31	why this works . Okay ? So Okay , let's
00:34	launch into this . So , so you were asked
00:36	the following questions . So we had the problem ,
00:38	you have to work out instantly 46 Times five or
00:44	you wanted to know that pretty rapidly . So the
00:47	way that I work this out , the method ,
00:48	I'm going to show you are going to launch into
00:49	it straight away is what we do is we have
00:52	this number here that we're multiplying by five , so
00:54	half of 46 . it's 23 . Then we just
00:57	multiply , it's about time we just put a zero
01:00	on the end . So 230 . That's how easy
01:04	this is . Right , Okay . So what we
01:08	did is we have our number and we just put
01:09	a zero on the end . What about another one
01:12	of these ? What have you got to ask the
01:13	question ? 106 tops five . So The same sort
01:20	of thing . We're going to have our number half
01:22	of 106 , which is 53 And we put a
01:27	zero on the end . Okay ? So the answer
01:30	530 . Pretty basic . Right . What about I
01:33	give you one more of these ? I'll show you
01:35	a slight variation with this and I'll tell you why
01:37	this works . So let's have a look at this
01:42	. So say you had the following one slight variation
01:44	might be a number that doesn't have just as easily
01:47	, which is 24/7 times five decades . Because Half
01:51	of 27 is not a nice even number . So
01:53	all you do is we're gonna look at the number
01:55	below this , which is 26 and we're gonna have
01:57	that number . Okay , so half of 26 ,
02:01	Well that's 13 . And you can see there's one
02:04	lot of five left over , so that one a
02:07	lot of five left over when it's going to add
02:08	that on there . Okay . So the answer 135
02:13	. So why does this work now ? This is
02:16	a really , really easy idea to think of why
02:18	this works when you think about it . Because realistically
02:22	, if we were to think about 55 is the
02:25	same as , say , 10 divided by two .
02:28	Okay , so what we're doing is we're dividing by
02:30	two first and then we're just multiplying by 10 ,
02:33	and that's why this works okay , because It's pretty
02:37	much the same thing as we've just got there .
02:38	Okay , we're dividing by two and times it by
02:40	10 . Okay , what about I show you the
02:44	25% how this works ? Okay . And look ,
02:46	I think you probably won't be that surprised once you
02:48	get there , started to get worked out . So
02:50	I say we were working with 25 and I wanted
02:52	to know what 36 Times 25 was . So straight
03:00	away , hopefully you can look at this answer and
03:02	you can say 36 . Now we're gonna be divided
03:06	not by two by four , So 36 divided by
03:09	four is nine And we're going to be multiplied by
03:12	100 . Okay ? So when you multiply it by
03:15	25 , We're dividing by four times 500 . Why
03:19	is this because 25 is the same as 25 ?
03:22	I'll put this in a different color . 25 is
03:26	the same mess . 100 divided by four . Okay
03:32	. So what we're doing is we're now divided by
03:34	four and times it by 100 . Okay , so
03:36	we've been multiplied by 25 . That's how you do
03:38	that one . Okay . What about we do another
03:41	one ? What about we do 64 times 25 .
03:49	Okay . So what do we do first ? We
03:52	Divide by four . So 64 divided by four .
03:55	Hopefully you're going to get the answer of 16 And
03:59	we're just putting these two zeros were multiplying by 100
04:05	. Pretty easy . Right ? Hey , what about
04:07	to just do one more of these a little bit
04:09	harder ? What about we do uh 17 Times 25
04:15	. And the reason I've chosen 17 because 17 ,
04:17	unlike these other ones , Four doesn't go nicely into
04:21	it . Like I have purposely chosen these other ones
04:23	to have four nicely go into them , but this
04:25	one it doesn't . So we still do the same
04:27	thing . We're divided by four . We're but we're
04:29	looking at the number which is below this at four
04:31	goes into which is 16 and that goes in four
04:34	times and you're going to see there's one left over
04:37	. So we're going to add this . So 25
04:42	. Okay . 425 . No problem . Right .
04:46	That's how we do those . Okay , so we're
04:48	just adding one group of 25 , it will say
04:50	18 . We had two groups of 25 or erupts
04:53	in 19 we had three groups of 25-75 to the
04:56	answer . So you sort of just going to keep
04:58	that in your head . Okay . Okay . What
05:00	about the last part now , which I said we're
05:03	going to cover , Which is as follows . It's
05:07	not rubbing it too quickly today . Okay . So
05:11	so we were looking at They're multiplying a number by
05:14	125 . So we had 72 Times 125 . Okay
05:25	, well before we start on that we might think
05:26	of 125 as being 1000 divided by eight . Okay
05:37	. Yes , 125 is five times 5 times five
05:40	. Eight years . Two times two times two .
05:42	There's a bit of a relationship there . Okay .
05:44	So um just just to keep that in mind if
05:47	you might pick that up , people were multiplied by
05:49	100 and 25 . Now we're going to be dividing
05:52	a number by eight and then times it by 1000
05:55	. Okay so you can think of any number you're
05:57	multiplying by you can actually change into a fraction like
06:00	this and if it makes it easier for you ,
06:01	Okay ? So 72 times 100 and 25 okay ,
06:05	We divide by 8 to 72 divided by eight is
06:08	nine . And we're going to multiply another by 1,000
06:12	9000 . That's your answer . Okay . What about
06:16	a bigger number now ? 112 times 125 . This
06:22	doesn't seem to want to work . So what is
06:23	right up there ? Okay , what's that equal ?
06:29	Okay So what we're gonna do , we're going to
06:33	divide our number here by eight . So let's do
06:36	that . So 112 divided by eight . Hopefully you're
06:40	going to get the answer of mm wrong color sticking
06:45	to the answer of 14 And we're gonna put two
06:51	three Zeros on the end . So 14,000 . Okay
06:56	, last of all , what about , you know
06:59	, a slight variance here . What about we did
07:00	42 times . 125 equals . That really bothers me
07:06	. Some of it tied it up by my hair
07:08	worse . Okay , so So we had this .
07:13	Now . What we can do is we're looking at
07:14	the number that eight goes into another number eight goes
07:17	into years 40 and it goes in five times .
07:21	Okay , so we keep 5000 and ahead of 51,230
07:26	. Is that we don't on here , but we've
07:27	got to lots of 125 left over to lots of
07:32	125 Is 250 . So that's how you do this
07:37	. Okay , so as long as you can keep
07:38	that in your head , it's all pretty good .
07:41	I hope you like that method . I actually use
07:42	this fairly frequently , especially that five times method and
07:45	that 25 method , I use that actually all the
07:47	time . Whenever I'm doing those sort of things ,
07:49	it's immediately how I do it . So anyway ,
07:51	hopefully that was of some help to you . Thanks
07:54	for watching . We'll see you next time . Bye
00:0-1	.

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