If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains ***.kastatic.org** and ***.kasandbox.org** are unblocked.

Main content

Current time:0:00Total duration:3:40

CCSS.Math:

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 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 all right 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 9 in the tens place but you could view this as 39 tens and eight the 10 that it is right directly above is exactly 39 tens and zero ones or 390 and then the next 10 would be if we go up to 40 tens 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 5 or greater so we would round the tens 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 it's again you could view this as 25 tens and one and 251 is clearly closer to 250 we can round down another way to think about it is you look at the tens place and then you look at the ones place and you say hey this is less than five so I'm going to round down to on the tens 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 then there's a lot of different ways to approach addition but you could say all right I have I can think about the hundreds I have for hundreds and I'm adding that to 200 so 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 tens and if I'm adding it to five tens well I'm still going to have five tens and then I have zero ones in either case 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