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### Course: 3rd grade > Unit 3

Lesson 2: Estimate to add multi-digit numbers# Estimating when adding multi-digit numbers

Let's learn about estimating when adding multi-digit numbers. We see how to round numbers to the nearest 10 and then add them together to get an approximate sum. This technique is helpful for us when we need to make quick mental calculations and don't require an exact answer.

## Want to join the conversation?

- When estimating, where do you round it? With the example on the video 398 + 251 ≈ 650 you round 398(a hundreds) on the nearest
**tens**which is 400 and 251(a hundreds) to 250. What if it's a thousands or tens thousand? Like 3981 or 25187?(35 votes)- The goal is to get close to the answer. How your estimate or round will depend on how accurate you want your guess to be (or what your math problem asks for). With your example, i would probably add 4000 and 25000 for an estimate of 29000(27 votes)

- What if it is in decimal numbers?(4 votes)
- For decimal numbers we usually just round to the nearest integer or to the n-th decimal place.

Let's say 3.14159265359.

We can round it to 3, 3.14 or 3.1416.

It depends on the problem statement or simply how precise you want it to be.(1 vote)

- help me it's nightmare foxy in my daycare and in my bedroom(1 vote)
- I don't get it help pls(1 vote)
- I believe it's like this: Let's say you get uh.. A bit lazy.. Or don't have much time, you could do the answer roughly. Example: You could do 467 + 789, but to estimate it, you could do it as 400 + 700. The answer would be 1100, please

correct me if this is wrong.(1 vote)

- so what is the real number?(1 vote)
- I do not know what I’m supposed to doing(1 vote)
- Can you tell me how 398+1 is 400 because he took 1 from 351 and he's saying it is 400(1 vote)
- Sal is saying that the number 351 rounds up, in terms of hundreds, to 400.
*Hope this helps!*(1 vote)

- There was a question in the quiz for this section which asked for 1 answer but had two correct answers you could give. It said something like what can be rounded to 200 in the hundreds place? And gave two acceptable answers but says u can only choose 1 answer.

One selection said 150 and 249.

The other selection said 150 and 240.

The third selection being eliminated, why don't both of the top two answers work?(1 vote)- This question is asking which two of the following numbers round up to 200.

Because 249 and 240 do not round to 200 while 150 does, the selection is incorrect as both numbers need to round to 200.*Hope this helps!*(1 vote)

- When estimating, where do you round it? With the example on the video 398 + 251 ≈ 650 you round 398(a hundreds) on the nearest tens which is 400 and 251(a hundreds) to 250. What if it's a thousands or tens thousand? Like 3981 or 25187?(0 votes)
- Then you can simply round to the nearest hundred to make it easier. With your example, it looks like this:

3981 rounds up to 4000 in terms of hundreds

25187 rounds up to 25200 in terms of hundreds

I can now estimate that 25187 + 3981 is close to ~29200 because 25200 + 4000 = 29200.*Hope this helps!*(1 vote)

- I don't understand :c(0 votes)

## Video transcript

- [Instructor] Let's say
that we run an egg farm, and yesterday, we went out
and we collected 398 eggs from the chickens, and then today, we went out and we collected
251 eggs from the chickens. And what we're curious about is, roughly how many eggs do we now have that we've collected
over the last two days? Well, there's a few ways we could do that. We could just add 398 to 251, and figure out what that is equal to. But let's say that we
don't have a lot of time, and we don't have paper, and
we want to do it in our heads, and we don't care about the exact number. We want to know roughly
how many eggs do we have? Well, that's where the tool
of estimating is useful. So instead of saying the
exact amount of eggs, we could say, how many do we roughly have? And that's what this
squiggly equal sign means, it means, hey, we don't have
to get the exact number, but how many do we roughly have? And so we will be estimating. And the way that we can do that is that we can round each of these numbers to a number that is close to them, but that's easier to add in our heads, and then we add those two numbers. So pause this video, and see
if you can do that on your own before we do it together. Alright, now let's do this together. Let's start with 398, and let's
round it to the nearest 10. So, 398, you can view this as, we have a nine in the 10s place, but you could view this
as 39 10s, and eight. The 10 that it is right directly above is exactly 39 10s and zero ones, or 390, and then the next ten would
be if we go up to 40 10s, which would be the same thing as 400. Now, which one would we round to? Well, 398 is closer to
400 than it is to 390, and we've talked about
in the rounding videos that you can see that this right over here is five or greater, so we
would round the 10s place up. So we would round to 400. And so, 398, we round to 400. And then we're going to add that to 251, what's that rounded to the nearest 10? So I'll do that again, right over here. 251. The 10 that is right below
that is going to be 250. The 10 that is right above that is going to be 260. Once again, you could view
this as 25 10s and one. And 251 is clearly closer
to 250, we can round down. Another way to think about it
is, you look at the 10s place, and then you look at the ones place, and you say, hey, this is less than five, so I'm gonna round down
to on the 10s place, so we get to 250. So when you round 251 to the nearest 10, you're going to get 250. Now, I encourage you, add
these two in your head. What would this be equal to? Well, the way that I
think about it in my head, and there's a lot of different
ways to approach addition, but you could say, alright, I can think about the hundreds. I have four hundreds, and I'm
adding that to two hundreds, and let me do this in a new color. So if I have four hundreds, and I'm adding it to two hundreds, well, that's going to be six hundreds. And if I have zero 10s, and
if I'm adding it to five 10s, well, I'm still going to have five 10s, and then I have zero ones in either cases, so I'm still going to have zero ones. And so there you have it. We were able to estimate 398
plus 251 as being roughly 650.