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# Missing number for addition and subtraction within 1000

CCSS Math: 3.NBT.A.2

## 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.