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

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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