Math Antics - Multi-Digit Subtraction - Free Educational videos for Students in K-12

Math Antics - Multi-Digit Subtraction - By mathantics

Transcript


00:03	Uh huh . Hi and welcome to Math Antics .
00:08	In this video , we're going to learn how to
00:09	do multi digit subtraction . It's similar to doing multi
00:13	digit addition like we learned in our last video but
00:15	there's a few important differences . The main differences with
00:19	subtraction . The order of the problem matters with addition
00:22	. You can switch the order of the numbers you're
00:24	adding and you'll still get the same answer . Five
00:27	plus two equals seven and two plus five equals seven
00:30	. But with subtraction , if you have the problem
00:33	five minus two , you'll get three . But you
00:35	can't switch the problem around . You won't get the
00:38	same answer if you try to do to minus five
00:41	instead . In fact , you'll probably get confused because
00:44	you'll be trying to subtract a bigger number from a
00:46	smaller one with multi digit subtraction . It's important to
00:50	remember that order matters , especially when you're rewriting your
00:53	problem . Often you'll be given a problem like this
00:56	38 minus 25 you'll have to rewrite it with the
01:00	number stacked up like we did with addition . But
01:03	you have to make sure that the first number the
01:06	one you're taking from goes on top and the number
01:09	you're taking away is on the bottom . Another hint
01:12	is that the bigger number should always be on top
01:14	. Okay , let's go ahead and try this car
01:17	. We've got 38 on top and 25 below it
01:21	. And the ones places are lined up just like
01:23	they should be . Now we draw our lines so
01:26	our answer can go below it and we write a
01:28	minus sign over here on the left to show that
01:30	we're subtracting . Now we can start getting our answer
01:33	. And just like with addition , we always start
01:36	with the ones place column . Here we subtract the
01:39	bottom number from the top 8 -5 equals three .
01:42	So the three goes in the ones place of our
01:44	answer . Now we move to the next place column
01:47	to the left . The 10s place There we have
01:50	3 -2 which is one there . We just subtracted
01:54	25 from 38 and found out that the difference is
01:58	13 . All right . Let's see another example .
02:01	1:35 -27 . Uh huh . This is where multi
02:06	digit subtraction can get a little tricky . Let's rewrite
02:09	our problem . 1:35 on top 27 below it ,
02:13	with the ones places lined up neatly and our answer
02:16	line and subtraction symbol in place there . Now we
02:19	can start subtracting . Oh look at this and our
02:23	once place column . The digit on top is smaller
02:26	than the digit on the bottom . How can we
02:28	subtract a bigger digit from a smaller one ? Did
02:31	we make some kind of mistake ? No , we
02:33	wrote our problem correctly . The bigger number is on
02:36	top . Sometimes this just happens . The top digit
02:39	might be smaller than the bottom digit . So you
02:41	can't subtract it unless you borrow . Here's how borrowing
02:46	works . The top digit is five but the digit
02:49	below it wants to take seven away . Sorry I
02:52	don't have seven . I only have five . Well
02:56	what about your neighbor ? He's in a bigger number
02:58	place . He's loaded so you can just borrow from
03:01	him . Excuse me . I've got a little problem
03:04	. Do you happen to have something I could borrow
03:07	? Why ? Of course . Here you go .
03:09	Great . That one will help . But if you
03:12	just add 1 to 5 you would get six .
03:14	But fortunately this one came from the next number place
03:18	and it really represents a 10 . And when we
03:20	add 10 to 5 we get 15 which is big
03:23	enough . Now instead of this column being five minus
03:27	seven it's 15 minus seven And 15 -7 equals eight
03:32	. Okay we've got the first digit of our answer
03:35	now we can move on to the next column .
03:38	But remember we borrowed from that number place ? It
03:41	used to be a three but now it's a two
03:43	. It went down by one because we borrowed from
03:45	it . Well remember we really borrow 10 because it
03:49	was in the next number place but it's sometimes easier
03:52	to just think of it as borrowing a one and
03:54	getting to stick that one in front of the digit
03:57	that needed to borrow . So in the 10s place
03:59	we have to -2 which gives us zero in our
04:02	answer . And then our last column just has one
04:06	minus nothing or one minus zero . So that's still
04:09	just one there . We have calculated that the difference
04:12	between 1 35 and 27 is one . Oh wait
04:17	all right let's try another example with borrowing or regrouping
04:21	as some teachers call it . Let's subtract 58 from
04:24	426 . Again we start by subtracting the digits in
04:28	the ones place calm here we have six minus eight
04:31	. And since six is too small to subtract eight
04:34	from . We'll need to borrow we always borrow from
04:37	the number place on the left . We'll borrow a
04:39	one which is really a 10 . And we'll write
04:42	it in front of our borrowing digit in this case
04:44	six which gives us 16 . And don't forget to
04:48	make the digit we borrowed from smaller by one .
04:51	You can just cross it out and write the new
04:53	smaller number above it like this . Okay now we
04:57	can subtract from the first calm 16 -8 equals eight
05:02	. Now for the 10s place since we borrowed from
05:04	this column it's become 1 -5 . But again the
05:08	top numbers too small so we'll have to borrow again
05:12	We borrow one from the next number place over which
05:15	means that digit will change from four to a .
05:17	three . Then we put the one in front of
05:20	the borrowing digit which will make it 11 . Now
05:23	we can do the subtraction for that column . 11
05:26	-5 equals six . And the last columns easy .
05:29	We bring that left over three down to the answer
05:32	line because there's nothing there to subtract from it .
05:34	And that means 368 is our answer . So now
05:38	you know the basics of multi-digit subtraction . But before
05:41	you move on to practicing with exercises I want to
05:44	show you one more important trick . Once in a
05:46	while you'll come across a situation where you need to
05:49	borrow from the next number place over but that digit
05:51	happens to be a zero . How can you borrow
05:54	from a zero ? Well you can't so you'll have
05:57	to borrow from the next two digits instead of just
05:59	one . In this case . Instead of borrowing from
06:02	zero borrow from 40 , you'll get the one you
06:05	need to borrow and the 40 will become a 39
06:08	. Or in this case if the three needs to
06:11	borrow , don't borrow from the zero borrow from the
06:13	20 and they'll be 19 left over . Or what
06:17	if there's two zeros in a row like this problem
06:20	? Well if the two needs to borrow then borrow
06:22	from the whole 500 next door . The tool become
06:25	12 and the 500 will drop by 1 to become
06:28	499 . Get the idea you can do that no
06:32	matter how many zeros are in a row , just
06:34	keep including the next digit to the left until you
06:37	get a number you can really borrow from . Okay
06:40	, that wraps up this lesson . Hopefully you have
06:42	a better idea of how multi digits attraction works ,
06:45	but to really get it down , you've got to
06:47	practice so be sure to do those exercises . Thanks
06:50	for watching . And I'll see you next time .
06:53	Yeah , learn more at math Antics dot com .

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