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## Arithmetic (all content)

### Unit 2: Lesson 18

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

CCSS.Math:

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? (Please respond)(15 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.(4 votes)

- Could you say this is kind of referring to 'fact families'?(8 votes)
- How do you make raibows(6 votes)
- easy! the sun reflecting the light on to the water from the rain.(3 votes)

- This still does not make sense(7 votes)
- I need Missing number for subtraction within 1000(7 votes)
- duskpin ultimate style avatar for user eleanor.brockelsby

eleanor.brockelsby(5 votes) - How do you locate the difference of numbers in your head? (Please respond)(4 votes)
- - [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.(4 votes)
- Okay, this isn't based on the video but I don't know what this question categorizes as. Please help me anyway!

So Alice and Bob play a game involving a circle whose circumference is divided by 12 equally spaced pedals. The points are numbered clockwise, from 1 to 12. Both start on point 12. Alice moves clockwise and Bob moves counterclockwise. In a turn of the game, Alice moves 5 points clockwise and Bob moves 9 points counterclockwise. The game ends when they stop on the same point. How many turns will this take?(4 votes) - can some one help me with this(3 votes)

## 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.