Estimating multi-digit multiplication word problems
About this video
A survey found 2,138 people took vacation photos with a camera and about 15 times as many used a phone. The video estimates with friendly numbers: it keeps 15, rounds 2,138 to 2,000, and finds 2,000 x 15 = 30,000, so about 30,000 people used their phone.
You'll be able to
- choose friendly numbers to estimate a multiplication.
- estimate the answer to a 'times as many' word problem.
- explain why an estimate is useful when an exact answer isn't needed.
Key words
- estimate
- A close, sensible guess worked out with easier numbers.
- friendly numbers
- Numbers that make arithmetic easy, like 2,000 instead of 2,138.
- approximately
- Close to but not exactly; written with a squiggly equals sign.
- times as many
- A comparison that means multiply, like 15 times as many.
Transcript
- We're told results from a survey showed that 2,138 people took photos with a camera when on vacation. About 15 times as many people took photos with their phone. About how many people took photos with their phone? So pause this video and take a shot at this.
And remember, we're just estimating. They're saying, about how many people took photos with their phone, not exactly. All right, now let's work through this together. So 2,138 people took photos with the camera and 15 times as many, and even this looks like an estimation, about 15 times as many people, took photos with their phone.
So you obviously could take 2,138 and multiply it by 15, which you're going to need paper for. Most of us need paper to do something like this. And if you're just walking around and someone's asking you to estimate, you might not have the paper or the time to do something like this.
And if you're just estimating, you just wanna say, okay, can I come up with friendly numbers? And what do I mean by friendly numbers? Friendly numbers are numbers that you make it a lot easier to do the arithmetic with. In this case, we're going to have to multiply.
And my head says, all right, 15 is already reasonably friendly and it's already an estimation, so I don't know if I necessarily wanna take an estimation of an estimation. And then I can look at the 2,138. And there's not always a perfect answer here. Sometimes you might wanna round it to 2,140, sometimes you might wanna round it to 2,100.
But for my head, it's a lot easier to multiply 2000 times 15 than 2,100 times 15 or 2,140 times 15. So that's what I'm gonna do. I'm gonna say, okay, about how many people took photos with their phone? It's going to be approximately, the squiggly equal sign is one way of writing approximately, 2000 times 15.
And that I can do in my head because two times 15 is 30. And I'm actually dealing with not two but 2000, so it's going to be 30,000. So roughly or about 30,000 people took photos with their phone.
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