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Course: Grade 5 (TX TEKS) > Unit 1
Lesson 3: Estimate multi-digit multiplication- Strategies for multiplying multiples of 10, 100 and 1000
- Multiply by taking out factors of 10
- Estimating multi-digit multiplication
- Estimate multi-digit multiplication
- Estimating multi-digit multiplication word problems
- Multi-digit multiplication estimation word problems
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Estimating multi-digit multiplication word problems
Use estimation as a strategy to solve multi-digit multiplication word problems. Created by Sal Khan.
Video 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.