Estimation word problems with mixed operations
About this video
A farmer has 347 apples, sells 153, then harvests 278 more, and we estimate how many she has now. The narrator rounds to friendly numbers (350, 150 and 300) and finds 350 - 150 + 300 = 500. He explains that estimates can differ depending on how you round.
You'll be able to
- round numbers to friendly numbers to estimate.
- estimate the answer to a two-step problem with subtraction and addition.
- explain why different estimates can all be reasonable.
Key words
- estimate
- A close answer that is easy to find, not the exact answer.
- friendly numbers
- Numbers that are easy to work with in your head, like 350 or 300.
- roughly
- About, or close to, but not exactly.
Transcript
- [Instructor] We're told a farmer has 347 apples. She sold 153 apples at the market, she harvests 278 more apples the next day. Estimate how many apples the farmer has now. So pause this video and have a go at this and then we'll do it together. All right, so now let's work on this together.
So in general, when people are asking you to estimate, you don't have to get an exact answer. So there isn't one right answer here, but it's usually a good sense that, well, you should just try to get these numbers closer to numbers that are a little bit friendlier, numbers that are easy to work with.
Maybe you can do the math in your head. And so I would take 347. That feels awfully close to 350. And if you think about it, it's even closer to 350 than it is to say 340. So I would say that 347 is roughly equal to 350. And then I would say 153. Well, 153 is roughly equal to 150.
And then last but not least, 278, let me do this in a different color. 278 is roughly equal to, we could go a lot of different directions with that, but I'll just round to the nearest 100 here. I'll say that this is roughly equal to 300. Now, why did I pick these numbers?
Well, these are pretty easy to work with. I could now say, all right, she starts with 350 apples and she sells 153 in the market, or I should say roughly 150. So another way I should say it is she starts with roughly, our estimate is 350 apples. We know the exact number, but this is our estimate.
She sells roughly, we estimate 150 apples, and then she gets another roughly 300 apples. And so what's 350 minus 150? You might be able to do that in your head. If you took away 50 from 350, you'd get to 300, and then you subtract another 100. This is going to be 200, and then 200 plus 300 is equal to 500.
So this isn't going to be the exact number. We could actually try to do the math here to figure out the exact number. But if I were to estimate how many apples the farmer has now, I would say it's roughly 500 apples. But you might have gotten a different answer.
You might have rounded maybe to the nearest 100 or done something different and you would've gotten a different answer here. But we're just trying to find directionally roughly an estimate of how many apples the farmer has.
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