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Estimating when subtracting large numbers

Use estimation to find reasonable solutions to 2- and 3-digit subtraction problems.

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Video transcript

- [Narrator] Let's say that you have a jar of jelly beans, and you know that there are exactly 282 jelly beans in that jar of jelly beans. And then the next day you come and you see there are fewer and you say, what happened, and let's say someone who lives with you or your friend says, oh yeah, I counted exactly and I ate 59 of those jelly beans. So you started with 282, 59 are taken out, and you don't need to know exactly how many jelly beans are left, but you wanna estimate roughly how many jelly beans are left, and that's why, as this title of the video says, we're going to estimate when subtracting large numbers. And we can debate how large is large, but these are reasonably large. So let's do that, let's see if we can estimate what 282 minus 59 is. And sometimes when we're estimating, we will use this squiggly equal sign. This means approximately, which is another, just a fancy way of saying roughly what is this equal to. So the way that I would approach this, I would say hey, are these close to numbers that are easier to subtract with. So for example, 282, I'd say, you know what, maybe I can round to the nearest 10 here, and so I could round it down to 280, if I round down to the nearest 10. If I rounded up, it would be 290, but 282 is closer to 280 than it is to 290. So I would round this, when I'm estimating to 280. And then what about 59? If I were to round to the nearest 10, what would I round that to? Pause the video and think about that. Well, 59 if I round down to the nearest 10, it would be 50, and if I round up to the nearest 10, it would be 60, and 59 is definitely closer to 60. So I would say that this is roughly, if I'm rounding, I could say, hey I'm gonna round that to 60. So I could say the 282 minus 59 is roughly going to be 280 minus 60. And now what is this? You might be able to do this in your head. You could view this as 28 tens minus six tens, and so if you have 28 of something minus six of it, you're going to be left with 22 of that something. And so you could say that this is going to be 22 tens. 22 tens, or another way to think about it. You have two 100s and then 8 tens, and then you're gonna take six tens away from it, so then you'll be left with two 100s and two tens. Just like we have here. So the whole point here, this is not, this is actually not exactly what 282 minus 59 is, but if you just said, hey ballpark do I, I wanna know, do I have roughly how many jelly beans do I have, well this could be useful. And you'll see later in life this is actually a very, very usefully skill where you might have these numbers that get a lot larger than this, but you'll round to the nearest 10, or you'll round to other places, so that you can do that computation in your head.