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# Missing number for 3-digit subtraction within 1000

CCSS.Math:

## Video transcript

let's say that we are told that 495 is equal to 621 - blank what would blank be pause this video and see if you can figure that out okay now let's do this together and one technique is to try to visualize this on a number line 495 is what you get when you subtract something from 621 so let me put those two numbers on a number line so let me do the number line like this so we're going to start at 621 and subtract blank to get to 495 so if this is 621 we're going to subtract some amount to get to 495 so let's say this right over here is 495 now what amount did we subtract so here we are subtracting blank that's my blank over there and so we need to figure out what that is so how do you go from 621 to 495 well one way that my brain does it first we can subtract 21 to get to 600 so let me do minus 21 would get us to 600 then we could subtract another hundred to get to 500 so then we could subtract another hundred so minus one hundred to get us to 500 and then to go from 500 to 495 we would subtract another five so to go from 621 to 495 and I picked these numbers intentionally I picked 21 to subtract to get to 600 I picked a hundred to get to 500 and then I picked five to get to 495 I subtracted a total of twenty-one then I subtracted one hundred then I subtracted five so I subtracted one hundred I subtracted twenty-one and I subtracted five and so how much did I subtract in total so let me put it this way this is the same thing as 621 minus these things that's equal to 495 so in total let's see if I subtract twenty one and I subtract five that is subtracting 26 and so I subtract one hundred and 26 so I subtracted a total of 126 so 495 is equal to 621 minus 126 let's do another example let's say we want to figure out if we if someone tells us blank - 435 is equal to 210 what is blank in this situation well this is a little bit different now because now we're not trying to figure out what to subtract from the larger number in this situation we're trying to figure out the larger number the larger number minus something is equal to 210 but as always it's useful to draw this on a number line just to make sure we're thinking about things in a reasonable way so we're going to start at blank take out 335 and then we're going to get to 210 so we're starting here maybe I'll put a question mark there so we know that point we're starting there we are subtracting 435 so that's minus 435 and we are getting to 210 so to figure out blank we could go the other way around we could start at 210 and then just add and then just add 435 so what's 210 + 435 well there's many ways of doing it by encourage you to pause this video and see if you can work that out well one way to do it is first we can add 400 I'll do this in another color just for a little bit of a variety so first I'll add 400 so if I add 400 plus 400 that would get us see if you had 400 to that that would get you to 610 and then if I add 30 to that so if I add plus 30 and I haven't drawn it exact but this is just to help us think about things that's three more tens and I already have 110 so 10 plus 30 is 40 so I guess this is 640 and then I'm going to add the five so this right over here if I add 5 to it is 645 so there you have it 645 - 435 is equal to 210