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### Course: 3rd grade > Unit 3

Lesson 8: Addition and subtraction missing value problems# Missing number for 3-digit addition within 1000

Sal solves for the missing number in problems like ___ + 261 = 630.

## Want to join the conversation?

- How do you locate the difference of numbers in your head?(30 votes)
- @Aiden Delgado: Taking the difference of numbers in your head requires lots and lots of practice. You have to memorize arithmetic (e.g. 4-1 = 5, 8-6=2) until you recognize the differences when you see them. Keep working on your computer or on paper until you are able do the arithmetic in your head.(17 votes)

- I dont understand the subtraction part of finding the missing number with the number line.

For example- ?-435=210

The answer I got was 225. But checking a calculator it shows up that 225-435= -210

When I ask for a hint on the quiz, it tells me the answer is 645-435-210

Using a number line, how can I find out that 645 is the missing number for

?-435=210 ?(3 votes)- I see what you did wrong. You subtracted 435 from 210, giving you a negative number: -225. That's not what you're supposed to do.

The problem is asking what number minus 435 will equal 210. So we automatically know the number has to be larger than 435. How larger? Well, it has to be 210 more than 435. If you're confused, consider the following, simpler equation:

? - 10 = 6

How would you solve something like that? You would add 10 to 6. That gives us 16. Our problem is true, because 16 - 10 = 6.

So do the same thing with our other problem. 435 + 210 = 645. Check with calculator, and we see that in fact 645 - 435 = 210.(10 votes)

- ? + 261 = 630 what is the question mark(1 vote)
- It's asking you what number plus 261 is equal to 630. The question mark is just standing for a number that we don't know yet.

To find out what it is... Remember that addition and subtraction are closely related to each other.(4 votes)

- do you still subtract to find the answer when it is 557(blank) equals 106?):(2 votes)
- Yes. For 557 - blank = 106, you would do 557 - 106 = blank.(1 vote)

- tysm sal your the best thanks for taking your time and putting your effort in to these videos your the best I think bc you take your time telling it my brothers on 2th grade I'm on 5th(2 votes)
- i would like a different strategy!(2 votes)
- I don't get the second step can someone explain to me it?(1 vote)
- can you tell me angain(1 vote)
- Doesn’t really make sense(1 vote)
- Anyone have an easier way to explain this(1 vote)

## Video transcript

- [Voiceover] So we have blank
plus 261 is equal to 630, and I encourage you to pause the video and see if you can
figure out what blank is. So there's a couple of
ways to tackle this. Let's visualize it on a number line. So that's my number line right over there. And we could start at blank, or I'll call that question mark. So we could start at question mark, and if we add 261 to it, so if we add 261, plus 261, we can get to 630. We can get to 630. That's the 630 right over there, which means that if we started at 630, if we start at 630, and we were to subtract 261, if we were to go the other way, if we were to subtract
261, so let's do that. So, minus 261. Minus 261. We're gonna get back to blank. We're gonna get back to our question mark. Now there's a bunch of ways
that you can compute this, but I'm gonna try to tackle it in a way that I would actually
try to do it in my head, or it'll simply my computation,
it's just one of many ways to figure this out. So the way I would do it is
I would first subtract 200, so I would first subtract 200. So that's about, I'm not
drawing it perfectly to scale. So I would subtract 200, so that's minus 200 right over there, which would get me to 430. I could do that in my head. I just subtracted two hundreds from that. And let's see, now I wanna subtract 60. So then I would subtract 60. So minus 60. Let's see, 430 minus 60. That's 43 10s minus six 10s. Well, that's gonna be 37 10s, or 370. This gets us to 370. And then we going to subtract one more. We have to subtract 260
and then we subtract one. So 370 minus one is 369. So this right over here is 360, 369. Now, another way we could have done it is we could have swapped the order. We could have said if
blank plus 261 is 630, we could have said 261 plus blank is equal to 630. All I did is I swapped the order here, but we could visualize
that as a different thing on the number line. So I'm going to, let me
draw my number line again. So there's my number line. And I could start at 261. So this time I'm starting at 261. 261. And I wanna get to 630. And I wanna get to 630, and the amount that I add
is going to be the blank. So let's see what I have
to add to get to 630. And I'm gonna add numbers that
are easy for me to compute. So I could add nine, I could add nine. Now, why did I add nine? Because that gets me to 270, which is a nice, round number. And then I could add,
let's see, I could 30 here to get to 300, so let me do that. So plus 30. I'm not doing this quite to scale, but this is going to get me to 300. And then from 300 to 630, well I could just add 330. I could just add 330 there. So how much did I add in total? This is 39 plus 330. That's 369. I added 369. So these are all ways of
getting to the same value, all ways of viewing the same problem.