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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?
• 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
• What if it is in decimal numbers?
• 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
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• I don't get it help pls
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• 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.
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• so what is the real number?
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• I do not know what I’m supposed to doing
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• Can you tell me how 398+1 is 400 because he took 1 from 351 and he's saying it is 400
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• Sal is saying that the number 351 rounds up, in terms of hundreds, to 400.

Hope this helps!
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• 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?
• 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