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Estimating quotients

CCSS Math: 4.NBT.B.6

Video transcript

- [Instructor] We are told to use estimation to tell whether each estimated quotient is reasonable or unreasonable. So we have a bunch of estimated quotients here. Someone is estimating that 2,419 divided by three is roughly equal to 7,500 and we have to figure out is that reasonable or unreasonable? And then we have two more quotients to figure out if it's unreasonable or reasonable. Pause this video and see if you can figure it out on your own before we work through it together. All right, now let's work through it together. The way that I'd like to tackle this is just try to do the estimation myself. I'll start with this first one. 2,419 divided by three. One way to estimate is try to think about what numbers are these close to that are a little bit easier to compute with. Some people would say they're a little bit friendlier so to speak. Well three is already a pretty simple number a pretty friendly number. 2,419, well it's a little bit more complicated. But I could try to think of is there there a multiple of three that's close to 2,419 that is a fairly friendly number? I do know that three times eight is 24. So if I had something like 2,400 which is the same thing as 2,400 that'd be easy to divide by three. So I could say that this is roughly equal to 2,400 divided by three. All I did is I said, hey 2,419 for estimation purposes let's just say it's roughly equal to 2,400. And then 2,400 is fairly straightforward to divide by three. This over here 24 divided by three is eight so 2,400 divided by three is 800. This is going to be 800. So 2,419 divided by three should be roughly around 800. They're saying that it's roughly 7,500. That's a lot more than 800. If they said 750 then I'd say, hey that's pretty reasonable. But they're saying 7,500. That's almost 10 times more than 800 so, this I would say is unreasonable. All right, let's think about this next one and again, pause the video and see if you can have a go at it. All right, I'm just going to try to estimate again. So 435 divided by five. Let's say that's roughly equal to. An easy multiple of five that's close to 435 if I wanna think of a friendly number it might be 400. So I'll say that's roughly equal to 400 divided by five. Now, we know that 40 divided by five is is eight, sorry. And so 40 10s divided by five is eight 10s. So this is going to be equal to 80. And you could think about it the other way. Five times eight 10s is going to be 40 10s which is the same thing as 400. This thing should be roughly equal to 80. Well, that's exactly what we estimated so that feels very reasonable. All right, this last one. Pause the video again, see if you can have a go at that. All right, so we have 6,202 divided by two. We could say that's approximately equal to or roughly equal to. Well, if I just wanted to do it very roughly I could say, hey 6,202 let's just go to 6,000 just to make things really easy. Divided by two. And so this is going to be six 1000s divided by two would be three 1000s or 3,000. But here they have 300. That's one tenth of what I have over here. So this is unreasonable. And we're done.