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# Missing number for 3-digit addition within 1000

CCSS.Math:

## Video transcript

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 - 261 we're going to get back to blank we're going to get back to our question mark now there's a bunch of ways that you can compute this but I'm going to try to tackle it in a way that I would actually try to do it in my head or it'll simplify 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'd first subtract 200 so that's about it's not 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 can do that in my head I just subtracted two hundreds from that and let's see now I want to subtract 60 so then I would subtract 60 so minus 60 so let's see 430 minus 60 that's that's forty three tens minus six tens well that's going to be 37 tens or three hundred seventy this gets us to 370 and then we got to subtract one more we have to subtract 260 and then we subtract one so 370 minus 1 is 369 so this right over here is 360 360 nine now another way we could have done it is we could have swapped the order we could have set it blank plus two 261 to 630 we could have said we could have said 261 plus plus blank is equal to 630 all I did is I swapped the order here but we can visualize that as a different is 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 want to get to 630 and I'm going to 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 going to add numbers that are easy for me to compute so I can 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 add thirty here to get to 300 so let me do that so plus thirty 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 so I could just add 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 always of viewing the same problem