Adding and Subtracting Decimals - Free Educational videos for Students in K-12

Adding and Subtracting Decimals - By Anywhere Math

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00:0-1	Welcome to anywhere Math . I'm Jeff , Jacobson .
00:02	And today we're gonna be adding and subtracting decimals .
00:05	Let's get started . All right , let's get started
00:25	. So example , 18.13 plus 2.97 . Uh The
00:30	trick with adding or subtracting decimals is that you always
00:36	always , always always line up the decimal point .
00:40	And the reason for that is that that makes sure
00:44	that you're adding the right place values . This is
00:47	to ones I want to make sure I add that
00:50	with the eight ones here . This is 7/10 .
00:55	I want to add that with the one tense there
00:58	. If I don't if I don't have the decimal
01:01	points lined up , I might be adding a 7/10
01:05	With an eight and then that would give me 15
01:07	but that's not right . This is only 7/10 .
01:10	That's eight holes . Right ? So that's why you
01:13	want to make sure you line it up . If
01:15	you have great handwriting . This should be no problem
01:18	. If you're a little bit messy , uh Maybe
01:21	you can kind of do some little tricks to help
01:23	you make sure that they're lined up . So uh
01:26	for example I will do 8.13 plus That's some points
01:33	here . So two points 76 Okay this is really
01:39	nice . They both have three digits so that makes
01:42	it really easy . But notice my decimal points are
01:47	lined up okay in your answer your decimal point will
01:52	be directly below the other ones . So I know
01:55	that my decimal point is going to be there .
01:58	Okay it's gotta stay in line . So now it's
02:00	simple . I just add like normal three plus six
02:03	is nine . 1-plus 7 is eight . There's my
02:07	decimal point 8 2 is 10 . So my final
02:11	answer is 10 .9 , sorry 10.89 . 1 thing
02:17	to do when you're dealing with decimals . When you
02:20	get your answer , check to see if it makes
02:22	sense . Right ? If you were gonna um round
02:27	these and add them you would say okay that's about
02:29	eight , that's about three . If I round up
02:32	that would be three . So eight plus three is
02:34	11 . My answers should be somewhere around 11 And
02:37	it is that's very close to 11 . If I
02:41	had the desperate point in the wrong spot , maybe
02:44	I had my desperate point here And I would get
02:46	108.9 . If I check to see if it made
02:51	sense with some rounding and estimation , I would realize
02:54	like Wow 11 I said it should be close to
02:57	11 and I got 108.9 that's not very close .
03:01	So then hopefully you can go back and see oh
03:03	yeah I made a mistake with my decimal point .
03:06	So that's example one . Let's try another example example
03:10	number 21.459 plus 23.7 . Uh The same thing I
03:16	got to make sure I line up my decimals when
03:18	I'm doing this edition and notice here we've got four
03:22	digits here and three here . So it's even even
03:24	more important than the last one that I line them
03:27	up correctly . Uh So 1.459 plus 23 23 .
03:38	Then my decimal point seven . Okay notice again my
03:45	decimal points are lined up like they should be .
03:49	So I'm adding my 7/10 gets added with the 4/10
03:53	here and this one gets added with the three .
03:58	That's in the ones place as well , so that's
04:00	good . That's what we want . Uh Now you
04:02	notice there's nothing here . How can I add this
04:05	with nothing ? Well with the most I can ,
04:08	as long as I'm to the right of the decimal
04:10	point , I can add as many zeros at the
04:12	end of that as I want and it's not going
04:14	to change the value . So what we do is
04:16	we just put a couple of zeros in there as
04:19	kind of placeholders just so everything lines up . Okay
04:22	and that's going to help us so we won't make
04:25	silly mistakes . So nine plus zero is still nine
04:29	. That's going to be five . 4-plus 7 is
04:32	11 . My decimal point goes there . I'm carrying
04:36	the one There's five and 2 . So I got
04:41	25.159 . And again let's just double check to see
04:45	if that makes sense . That rounds to about one
04:48	that would round 2 20 for one plus 24 is
04:52	25 . My answer should be around 25 which it
04:56	is so I am happy with that . Okay ,
05:00	let's try another example . Alright example # three now
05:04	we're subtracting 21.9 -1.605 . Same process . Line up
05:10	the decimal points first and then we will go ahead
05:13	and do the subtraction . Uh so 21.9 that has
05:17	to be first because The order matters for subtraction .
05:22	So 21.9 One point . So the ones going here
05:27	point my decimals are lined up 605 . Make sure
05:34	everything looks nice and organized . My decimal point are
05:40	lined up so that's good . Uh And now I'm
05:43	going to subtract just like in the last example you
05:46	can see well there's nothing here . How can I
05:49	subtract five from nothing ? Well like we said earlier
05:53	we can put zeros as placeholders to help us with
05:57	our subtraction . So I'm gonna put two there .
05:59	So now we have The same amount of digits on
06:02	the on the right of the decimal point . So
06:05	now here we go . 0 -5 . I can't
06:08	do so I gotta borrow I can't borrow there .
06:10	So that becomes a that becomes 10 borrow again that
06:15	becomes nine that becomes tense . So now I'm good
06:18	. 10 -5 is five 9.0 is nine . 8
06:24	-6 is two . Decimal Point stays in line with
06:28	the others . 1 -1 is zero and 2 -
06:33	. I could put a zero here if I wanted
06:34	but two minutes nothing is too okay last let's check
06:38	to see if it makes sense . With our estimations
06:41	. This would round 222 1.605 with round 22 So
06:46	20 to minus two is 20 . Our answer should
06:50	be around 20 and it is so we are happy
06:55	with that . Here's some to try on your own
07:09	. All right , here's the dice example . Uh
07:11	we got a word problem . Your meal at McDonald's
07:13	cost $3.45 . Your friends meal is $3.90 . If
07:19	you pay for both . Uh with a $10 bill
07:23	, how much change should you get back ? So
07:26	uh if you think about it , you're gonna have
07:28	to take well whatever the cost of both meals together
07:32	is and subtract that from 10 . So if we
07:35	write an expression is going to be $10 The sum
07:42	of your two meals . So $3.45 plus $3 and
07:50	90 cents . Right ? You would add both your
07:53	meals up to track that from 10 and that's gonna
07:55	give us our change . So let's see . Um
08:00	3.45 plus 3.90 Let's do that over here to the
08:04	side . 3.45 plus $3.90 . That gives me 5
08:14	13 . They're small points are in line , There's
08:19	6 7 . So I got $7.35 total . Uh
08:24	And that seems about right . So now I'm gonna
08:27	do 10 minus that total Which was $7.35 . Okay
08:35	, now I can do that subtraction . I'm gonna
08:37	do it over . Well let's see , I'll do
08:39	it right here . So I had 10 -7.35 .
08:45	Now I'm going to subtract the whole number with a
08:47	decimal , but if you look there is no decimal
08:50	point there , right ? How can I do that
08:52	? How can I lined up the decimals ? If
08:53	there isn't one ? Well , with any whole number
08:57	I can add a decimal to the right of it
09:02	And then I can put as many zeros as I
09:04	want at the uh to the right , so I'm
09:06	gonna do that 10 . That doesn't change the value
09:09	. Still 10 um minus seven 0.35 Then like I
09:17	said like we did before , I'm gonna add my
09:19	zeros as placeholders and they just do this attraction right
09:25	? My decimal points are lined up . Can't do
09:27	that , can't do that , can't do that ,
09:28	I gotta borrow from here . So that becomes zero
09:31	, that becomes 10 borrow again , that becomes nine
09:35	, that becomes 10 Borrow Yen nine and 10 .
09:39	So now 10 -5 is five . 9 -3 is
09:44	six decimal point And 9 -7 is too . Okay
09:51	so finally I got $2.65 cents change If I paid
10:01	for both with a $10 bill . And let's see
10:04	does that make sense logically We have both were about
10:07	$7 . 10 - About $7 . Should give me
10:11	about $3 change , and that's pretty close to $3
10:15	. Uh Here's one more to try on your own
10:28	as always . Thank you so much for watching ,
10:30	and if you like this video , please subscribe .

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