Fast math trick for adding a series of numbers - Free Educational videos for Students in K-12

Fast math trick for adding a series of numbers - By tecmath

Transcript


00:0-1	Good day . Welcome to Tech Math channel . What
00:01	we're going to be having a look at this video
00:03	is we're going to be having a look at when
00:06	you're shopping , working at a rough running total about
00:09	how much money you're spending . Okay . Um This
00:12	is uh mainly in response to me going down the
00:15	shops the other day and not actually having enough money
00:19	in my account . Ok . It's sort of prompted
00:20	me to think , okay , occasionally I have walked
00:23	through the supermarket and has to do a bit of
00:25	a running tally about how much of being how much
00:29	will be spending because otherwise , what happens is I
00:31	get to check out . I have no idea and
00:34	I get declined so I don't think anyone wants that
00:38	, so it's embarrassing . Uh Let's have a quick
00:41	look at how , you know , we can use
00:43	estimation to work out how to work out how much
00:47	roughly we should be paying to say you go to
00:49	the supermarket and I'm going to give a big long
00:52	list of things here um to say we have something
00:56	at $6.99 . Uh What about $7.20 ? Um 43
01:03	cents . We'll have something which is another 6 99
01:06	1 and I love the way the 6 99 or
01:08	7 99 . I know we all know what they
01:10	do that . Uh Okay , what about a dollar
01:14	23 $2 51 . Uh $6.20 $3.80 A dollar $25
01:32	, $2.35 what about 76 cents ? Another one which
01:39	is only 63 cents And something which is only 15
01:44	cents . Let's add all these together . If I
01:48	do this straight away , you might go because number
01:51	one you don't necessarily gonna walk around with a calculator
01:53	even though you'll have your phone on you and you
01:55	could add like this . But mentally the way that
01:59	I would be doing this is as follows , I
02:01	would be just walking through and keeping a rough running
02:04	teller using estimation estimation works on this . We're going
02:07	to be a rounding to the nearest most significant figures
02:11	. In this case we're going to be probably talking
02:12	about dollars . Okay . So Anything we're gonna be
02:16	looking about these sensing and then working out which is
02:19	closest it's closest to okay , so it's anything above
02:23	50 cents and above is going to be rounded up
02:25	to the next dollar above or anything below is going
02:28	to be rounded down . So this one is closest
02:30	to $7 , this one's closest to $7 . Okay
02:35	, This one's closest to $0 and so on and
02:37	so forth . But when you're walking through a supermarket
02:40	also you actually are not just writing things down .
02:43	So you're gonna be rounding things and adding them to
02:46	a running total . So we'll start off with $7
02:50	And then we're going to be adding another $7 .
02:52	So I'm going to be having a running total of
02:53	$14 . This is closer to zero . So we're
02:56	going to keep , this is $14 , this is
02:58	now closer to seven . So we're gonna up to
03:00	$21 . $21 . This one here is closer to
03:05	a dollar , $22 . This one is closer to
03:08	$3 for $25 . This one is closer to $6.31
03:14	dollars . This was closer to $4.35 dollars . This
03:18	one is closer to $1.36 dollars . This one is
03:21	closer to $2.38 dollars . This one is closer to
03:25	a dollar so $39 , this one's again closer to
03:31	a dollar so $40 . And this one is closer
03:35	to actually nothing closer to no dollars . And also
03:37	we're going to keep our it is $40 would add
03:40	nothing to our total estimation Is a $40 bill there
03:48	. Okay . And that's the way that I would
03:51	be uh estimating with these . I'll tell you what
03:54	I'd also be probably doing with it to tell you
03:56	the truth , I might even keep a buffer zone
03:59	of around about 10% . Okay , just to be
04:03	sure , okay , you don't want to get there
04:05	and then still be a little bit short . 10%
04:07	is really easy to work out because 10% just the
04:09	same as dividing by 10 . So 10% . Maybe
04:13	I'd be keeping a An extra $4 buffer divided by
04:17	four added on an extra buffer of maybe $10.44 .
04:23	Okay , pretty easy . Right . What about I
04:27	give you a bunch of examples with this and we'll
04:32	do this one in dollars also . Then we'll move
04:35	to some different currencies because different currencies present different problems
04:40	and that sort of deal but nothing huge . So
04:43	let's do another bunch of ones to say this was
04:48	$3.60 . I'll tell you what , see if you
04:50	can actually rarely as often add them as we go
04:53	. Okay . 3 60 4 20 51 cents ,
05:05	15 cents dollar 30 $2 55 $3 28 dollar 99
05:25	dollars 69 $2 I . E . D . $3.05
05:37	dollar 39 8 35 nine 36 Abdullah 81 . Okay
05:51	. So how did you go did you manage to
05:53	keep a running total on those ? Still doing it
05:56	? Okay . What about we do this right now
05:59	. So Again anything with this is 50 cents or
06:03	above ? We go up to the next dollar .
06:06	If it's below we go down and we leave it
06:08	as it is . So we have $4 bite tones
06:15	. $9 . Okay . $9 . $10 $13 $16
06:26	$18 . We're gonna make that into a two .
06:30	So it's $20 Into a three . So $23 .
06:35	$26 . $27 . Okay . This is gonna be
06:42	raring to an ice . So it's going to come
06:44	30 $5 . It's gonna become a nine because so
06:49	this is gonna be $44 . This is a two
06:53	. So it's $46 . Okay . Now with this
06:59	, you want to say look divided by 10 .
07:02	This is uh this is roughly $46 divided by 10
07:06	. Uh $4.60 quite $5 . So maybe we roughly
07:10	one . About $50 . Okay . 50 $51 .
07:15	Again , it's an estimation . So it's around about
07:18	. Okay . So , how'd you go with that
07:20	? All right . All right . What about I
07:24	try I'm gonna do one in a different currency ,
07:27	one of my favorite currencies around um when I travel
07:31	. And there's a reason I really like shopping in
07:33	this particular country and say which is India because what
07:38	they do on the in India is actually write the
07:40	prices of things on the back , on the actual
07:42	goods that you're buying a lot of time . What
07:45	I mean by this is it doesn't matter which store
07:47	you go into that item has the actual price written
07:51	. And it's really , really cool . So say
07:53	we're talking rupee um and we had things that were
07:56	costing ₹67 ah ₹95 . ₹120 . ₹175 . We
08:11	might do something which is uh ₹380 . Um 409
08:21	. Rupee . ₹199 . Uh ₹75 . 15 ruby
08:31	27 Rupee . 300 and ₹80 and 450 five Rupee
08:48	. Okay , So , the way that we might
08:50	be adding these together , see how you go ,
08:52	see how you gave . You're probably maybe you're in
08:54	India and you're going , okay , I can do
08:56	this . Or maybe you're traveling overseas and you're in
08:59	. See how you go . All right , So
09:01	this is going to be I'm gonna be making these
09:03	roughly to the tent . Okay , So let's do
09:12	That . Actually . Let's go just to the hundreds
09:15	. Okay . We got a lot of hundreds .
09:16	1 , 2 year . Let's make them to hundreds
09:17	. So , this is 100 , 200 300 .
09:25	This one is close to 500 . This one is
09:28	going to be Now . It was 400 and 900
09:32	1300 1500 . 1600 1600 1600 2000 2000 500 .
09:50	2500 , Ruby . All right . How do you
09:52	go with that ? Okay . We're going to add
09:54	to the tents for a second there , but no
09:56	, obviously hundreds of a much more significant figure on
09:58	that one and a much more , you know where
10:01	we're gonna be we're looking at . So yeah ,
10:03	2500 is our buffers and we'll have a 10% buffer
10:07	zone . So , maybe we're looking at we want
10:09	probably want to have 2750 honest . Just to be
10:13	sure when we're getting headed up , so don't be
10:15	surprised if it heads up that or maybe it heads
10:18	up Below is only 2250 . All right . All
10:22	right . One last example to say we were adding
10:27	up last of all , and this is an example
10:35	here . So say this this is what happened to
10:37	me the other day I was Shopping uh and I
10:40	was actually buying some furniture and things and that was
10:43	fairly , you know , it's fairly expensive to buy
10:45	furniture in Australia . Um so basically the prices they
10:49	were giving me with things like $399 , $1699 .
10:56	I was buying a bunch of things . I haven't
10:58	bought furniture and so long so everything is sort of
11:01	falling apart . The kids have wrecked it . Uh
11:03	1399 . I was also looking at a bunch of
11:07	chairs which were $69 each , and there were six
11:11	of them . So first off when I was trying
11:14	to work these out what I'd be doing as follows
11:16	. Obviously I'm going to be looking with this probably
11:18	at the nearest thousands , but I think the easiest
11:21	one to work out , Uh let's just go as
11:24	we go , this is roughly 400 to that .
11:27	We're gonna be adding 7 800 . So we're gonna
11:28	have 21 . 2000 100 . Okay , I'm just
11:34	going to be doing a running tally says 14 .
11:37	So we're going to be looking at 30 500 .
11:42	And then to this , this is roughly 77 6
11:45	is a 42 . So roughly 400 . I'm going
11:47	to be adding that on 3900 roughly . Okay .
11:56	And that's roughly what our prices there . Uh and
11:59	actually what the sales lady then said straight away as
12:01	they do it . Every furniture store in Australia probably
12:04	throughout the world as I said . But but today
12:06	we can offer you 20% off . Okay , well
12:10	how would have worked that out ? I'm just adding
12:12	a little bit of stuff in there because this is
12:14	what happened . Well , First off , 10% is
12:17	going to be this divided by 10 , so I'm
12:19	just going to get rid of the Zeros . can
12:21	be twice as much , so 20% is going to
12:23	be 700 80 or roughly let's stick in the roughly
12:28	zone of 800 . So we're gonna be looking at
12:33	800 coming off , we're looking at roughly $3,100 .
12:40	Okay . So that's the way that you can go
12:42	along and you can do this mental math in your
12:44	head and be working stuff out as you go along
12:49	. So it's through this rounding anyway . Hopefully that
12:53	was some help to you . We'll see you next
12:55	time .

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