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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.
• 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)
• Can you tell me how 398+1 is 400 because he took 1 from 351 and he's saying it is 400
(1 vote)
• In some of the practice problems. They round up by 5s for some reason. For example: I would estimate 383 into 380 but the practice problem expects me to round that up to 385. Similarly, if the number is 389, they tend to round it up to 400 rather than 390. This isn't always the case, but I feel like I'm missing something important here.
• 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?
• I don't understand :c
• help me it's nightmare foxy in my daycare and in my bedroom
• why is the equal sign changed when estimating? Is there a serten reason? Why does this happen in the process of math is all I'm asking? I also have the same question as Khanclifford but I don't understand your answer back could u simplify your answer for me?
• Because the value we get isn't the exact result, rather an estimation. We use ≈ to denote it.
The symbol ≈ means approximately equal to.
(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?