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Course: 3rd grade>Unit 3

Lesson 5: Estimate to subtract multi-digit numbers

Estimating when subtracting large numbers

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

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• I'm confused about how to estimate now. In the video, we are estimating by going up or down by 10s if the one-place value is greater or less than 5. - . So how come when estimating certain numbers that doesn't apply?

For example, during "Estimate to subtract multi-digit whole numbers" after the video numbers like

706 is estimated to be 700, not 710.
678 is estimated to be 700, not 680.
294 is estimated to be 300, not 290.
273 is estimated to be 300, not 270.

I thought when estimating you were only rounding the number up or down by 10 if the one place is lower or above 5 similar to how it was demonstrated in the video.

That could possibly be an error but if it's not can someone explain to me why certain numbers aren't rounded up or down by 10s but estimated by 100s and why some 10s digited numbers are rounded up when lower than 5 and vice versa? That would be good because that part wasn't talked about in the video.
(21 votes)
• In this video, the numbers were rounded to the nearest 10, but in the examples you gave, all the numbers are rounded to the nearest 100.

So although 706 ends in 6, we still round down when rounding to the nearest hundred because 6 is closer to 0 than 100.

Whether you round to the nearest 10 or 100 will depend on how accurate you want your estimate to be. Rounding to the nearest 10 is more accurate, but rounding to the nearest 100 may be easier.
(11 votes)
• Is it cumpulsory to use a curved equal sign?
(4 votes)
• Yes. The curved equal sign ≈ means "approximately equal to". You are required to use this since the value you evaluated isn't the actual value, but instead simply an approximation.
(8 votes)
• how do round manes numbers
(5 votes)
• is life real
(3 votes)
• yes it is real
(2 votes)
• Pay attention to through
(4 votes)
• I do not under stan
(4 votes)
• how would we round by 5?
(3 votes)
• I do not get it
(2 votes)
• How!? You should pay attenttion in class!?
(1 vote)
• what is lesson 5 about
(2 votes)
• I know it's confusing, but you'll learn from the video.
(2 votes)

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.