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Estimating adding and subtracting 3-digit numbers

Use estimation to find reasonable solutions to addition and subtraction problems.

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Video transcript

- [Instructor] What we're going to do in this video is get some practice estimating adding and subtracting three digit numbers and so here it says 398 plus 251 is, and this squiggly equal sign means we just need to make an estimate. We wanna know what is this roughly equal to? So normally I say pause the video but here I want you to work it out in your head. So I'm just gonna pause. I'm just gonna stop talking for like a second and see if you can come up with what this is or maybe I'll give you three seconds 'cause the whole point here is to estimate, not get the exact right answer. Okay, so I'm assuming you've had a go at it. Now, I'll tell you how I think about it. We of course could maybe take out some paper or you might even be able to get the exact answer in your head but in everyday life, if you're at the supermarket and you're like, okay, I guess it would be an expensive supermarket, something is $398, something else is $251. Roughly how much would I be spending if I bought both of them? Well, my head would say, alright, 398, I would round to nearest 10 at least, maybe the nearest 100. So let's round to the nearest 10. In the case of 398, the nearest 10 is the nearest 100. And so we would get to, this is approximately 400. And then this is, 251 is approximately 250. So if you were to add these two things up, you're going to get something that is approximately equal to 650. And lucky for us, that is one of the choices and what I'm about to do is not estimate but just see how far off we were. So if we were to just actually do it, 398 plus 251. Eight plus one is nine. Nine plus five is 14. This is 14 10s. We put four 10s here, and the other 10 10s we regroup as 100 and then one plus three plus 200 is 600. So in this case, in this particular case, we got awfully close with our estimation and if you don't need to get the exact number, well, that'll be fine. Let's do another example. In here, we are going to do a little bit of subtraction. So once again, pause this video or try to do it as quickly as you can. Try to estimate what this difference is going to be. 678 minus 273. Alright, let's do it together. So there's a couple of ways to tackle it. You could round to the nearest 10 and you could say, well, this is approximately 600, let's see, to round to the nearest 10, you would round up right over here so you'd say this is 680 minus you could say 270 which is going to give you and here you're still gonna have to do some mental math. This would be what? 600 minus 200 is 400 and 80 minus 70 is 10. So that's one way to do it or you could do it even rougher. You could just do to the nearest 100. So you could say, look, 678, the nearest 100 is 700 and then minus 273, the nearest 100 there is 300 and so this is even rougher but it gets us actually still similar answers and either way we estimated, either the kind of rougher way or the slightly less rough way, we got to something around 400. So you could feel good that this is your choice. The reason why we even have to make multiple choice questions for estimation is not everyone is gonna get the exact same estimate but the correct choice should be the one that any reasonable estimate would be closest to.