Percentage Math Trick 2 - Solve percentages mentally - percentages made easy - Free Educational videos for Students in K-12

Percentage Math Trick 2 - Solve percentages mentally - percentages made easy - By tecmath

Transcript


00:0-1	good day and welcome to the Tech mouth channel .
00:02	What we're going to be having a look at in
00:03	this video is a way of working out percentages of
00:05	an amount mentally . Okay . So anyway , without
00:09	much further ado , I'll give you an example .
00:11	So I'll give you five seconds to answer this one
00:14	. Can you do 12 of 80 . Okay .
00:21	Making your time's up . Hopefully you've got the answer
00:24	of 9.6 . If not , I'm going to show
00:27	you a really , really great little method of our
00:29	working these out okay , in your head . So
00:33	I'll show this way this works and this works around
00:35	the idea first off of what percent is now percent
00:39	. I think it's really important that we got an
00:41	idea of this percent means quite literally out of her
00:47	like a divided by and sent means 100 like 100
00:52	cents in a dollar or 100 years in a century
00:55	. Okay , so percent we're talking about a number
00:58	out of 100 to this . 12 is literally 12
01:01	out of 100 . Okay , it's a 12 out
01:04	of 100 parts for 80 . Okay , so we're
01:08	going to be using this idea to work out Our
01:11	percentages of in a number and there's a couple of
01:13	key percentages we're going to be looking at . First
01:16	off , we're going to be looking at 100% .
01:19	We're going to be looking at 50% of a number
01:22	. We're going to look at 10% of the number
01:25	. Well , look at 5% of a number and
01:28	1% of a number . And if you can pretty
01:32	much starting to automatically start to work these out for
01:35	another and they're not too bad , you can work
01:37	out percentages that are more complex . So , a
01:42	way of working at 100% 100% of number , 100
01:45	parts out of 100 of a number is the number
01:47	itself . So 100% of say 80 Is 80 ,
01:51	of 60 for his 60 400% of 27 , is
01:56	the number itself . So that's a nice as he
01:59	wanted to start off with 50% . 50 parts out
02:03	of 100 is a half of the number . Okay
02:06	, so half of 80 is 40 . Okay ,
02:11	So we're working at 50% of 70 would be half
02:14	of 70 . Okay . 10% of a number is
02:19	literally 10 parts out of 100 . No way we
02:21	can work this out very easily is divided by 10
02:24	. And the way we can do this really ,
02:26	really easy is we start with the decimal place ,
02:29	which is , hey , it's not shown and we
02:31	move to the left one time . So 80 becomes
02:35	eight . If it was 50 , would move the
02:38	decimal place back or divide by 10 and we would
02:40	get five , 5% . It's probably one of the
02:44	trickier ones but it's not too bad . It's ,
02:47	you can think of it a couple of ways .
02:49	What I typically think about it is it's this 50%
02:53	number divided by 10 . So we could get this
02:56	decimal place and move it back and it would become
02:58	for another way you might think about it is here
03:01	where you're just you're moving this decimal place back one
03:04	and then having it . So it's going back ,
03:06	you know , it's become eight and then you're having
03:08	it becomes four . So let's move the decimal place
03:11	back one time and having it . All right and
03:13	then we have 1% . Now 1% is one part
03:16	out of 100 . And to do this , we
03:18	divide by 100 . We move the decimal place back
03:21	not once but two times . So we go back
03:24	12 times . So 1% here is point out ,
03:29	okay , 1% of 70 is 700.71% of 120 would
03:35	move it back twice . So we have 100 and
03:37	20 would be 1.2 . Are you okay with that
03:41	? So the way that we use this to work
03:44	out this particular number is as follows . So 12%
03:47	we could think of as being one times 10 and
03:51	two Of these . 1% . Okay . And we
03:56	could really really easily keep this in our head two
03:59	times 1% or two times .8 Is 1.6 . So
04:04	we can keep this 1.6 in ahead and we can
04:07	add it to this 10% here . So 1.6 plus
04:12	eight . And it gives us Your answer . 9.6
04:17	. Okay . We don't we won't have to write
04:19	things obviously out like this all the time . This
04:22	is just showing you how you mentally start working them
04:25	out . So what about you give the next one
04:28	to go ? I recommend at the start you do
04:30	write them out possibly . Uh you know just just
04:33	to get it through your head a bit and then
04:35	what we'll do is we'll you start trying to work
04:37	it out mentally . Okay , so What about we
04:41	do another one , what about we do uh 51
04:49	of 17 . Can you work that out , pause
04:55	the video and then come back . Okay so hopefully
04:59	a pause it hopefully have worked it out . So
05:01	hopefully you also got the answer of 35 0.7 .
05:08	All right now let's work out how we got this
05:11	answer . So We have a 50% part Which is
05:17	a halfway which is 35 And a 1% part which
05:22	we move the decimal place back from here one two
05:27	times . So the .7 we add these together ,
05:32	we get 35.7 . Yeah . Or can you can
05:39	do that in your head . So what do you
05:42	want to give you another one ? What about we
05:44	do this one here ? We do 98% of 680
05:57	. So pause the video , Try to work it
06:00	out mentally this time and then work it out on
06:02	paper and see how you go . So did you
06:07	get the following did you get this ? This is
06:09	quite a difficult one . Did you get the answer
06:11	? 666 0.4 . Okay , so this way is
06:18	that this is probably the easiest way here . You
06:20	start with 100% and you've got two times 1% that
06:26	we're getting rid of . So we have 100% .
06:28	We're going to get rid of these two times 1%
06:31	. So 100% of society is society 1% is going
06:38	to be so we move the decimal place back 1-6.82
06:42	of these are going to be twice as much as
06:44	two times 6.8 . Yeah , So that's going to
06:48	be 38.6 . We're going to take that away .
06:53	So 38.6 , sorry , 680 takeaway 13.6 Gives us
06:58	area to hear . 666.4 . Okay . What I'll
07:03	do is I'll give you a whole bunch of this
07:05	. I'll give you what about we give you three
07:08	of them to work out then ? What I would
07:10	recommend you do try to work them out in your
07:12	head . Try to work the man on a bit
07:14	of paper using that way . See how you go
07:17	. All right , we'll go through the answers .
07:19	So what about this one ? Will give you 21
07:23	of 60 ? I'll give you this one . percent
07:30	of 45 . And what about this 1 ? Of
07:40	440 . Okay , pause the video , give them
07:45	a go . Okay , So how did you go
07:50	? Let's go through the answers here . So this
07:52	21% you might be thinking this a couple of ways
07:56	, but what you're probably going to be looking at
07:58	with this ? Okay , you're probably gonna be looking
08:00	at it to 10% as well as 11% 1% .
08:11	So The way we have that is to 10% is
08:15	going to be we move the decimal place back one
08:17	so six so two of those is going to be
08:19	12 . So 12 , keep that in your head
08:22	, I'll put that there and then we're going to
08:25	have the decimal place going back twice to get this
08:27	one sounds so 12.6 12 plus 120.6 Is 12.6 .
08:34	If you got that , you got the right answer
08:36	. Hopefully you did . Okay , what about we
08:39	do the next one and We get 42% of 45
08:46	. So this is a bit of a harder one
08:49	. The way I'd be looking at this one ,
08:50	I'd be thinking 42% . I'd be thinking well we
08:53	have 10% and I actually think there's four of these
08:58	. Okay , so four of these and 1% .
09:06	We have two of them . Okay , so the
09:10	way I would be doing this now , I'll show
09:12	you my working at how I'll do it but this
09:14	is not too bad to try and remember mentally .
09:17	Okay so we're gonna start with 10% of this is
09:20	just gonna be decimal place will move to here .
09:24	Are you going ? So we have four of those
09:28	so four times 4.5 , Double and double it again
09:31	. We get 9:18 And then 1% . okay the
09:37	1% that decimal place is going to , I didn't
09:41	mean to get rid of that , Put that back
09:43	there . 1% . We're going to move this decimal
09:45	place back twice . So 1 2 And we have
09:49	two of those 2.45 times two is 0.9 . So
09:55	we're gonna add that on . We can got that
09:57	color point no it 18 point on is our answer
10:04	. Okay let's do the very very last one .
10:08	Hopefully you got this one right . Hopefully you're going
10:10	really really good with them . Okay , let's see
10:13	, so 50% 6% of 4 , 40 I think
10:17	this is probably the hardest one so far . So
10:20	the way that I would be looking at this one
10:22	is we have a 50%, , just what we have
10:27	of 5% Which is going to be nice and easy
10:29	to work out because it's going to be 1/10 of
10:31	this . 50% here . We're going to have a
10:34	1% , only up to 56% . So 50% of
10:38	this is half . So to 20 5% were moving
10:43	the decimal place on this one . Back one ,
10:45	so 22 and 1% removing the decimal place here back
10:49	twice . 12 So 4.4 , we add all these
10:54	together so to 20 plus 22 plus 4.4 and hopefully
10:59	the answer again is 246 0.4 . Okay , that's
11:08	a really , really great little method to our work
11:10	. These are mentally in your head , so I
11:13	recommend you you write yourself up some , you try
11:16	to work them out and you might even get a
11:18	calculator or whatever to check your answers . Okay ,
11:21	That's not a bad idea when you're first starting to
11:22	do these , but hopefully that that method helps you
11:25	out a little bit anyway . Thank you for watching
11:27	. We'll see you next time .

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