Estimating multi-digit multiplication word problems | Grade 5 (TX TEKS) | Khan Academy

We're told results from a survey showed that 2,138 people took photos with the 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. 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. Even this looks like an estimation—about 15 times as many people took photos with their phone. 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.

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. If you're just estimating, you just want to say, okay, can I come up with friendly numbers? 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 just want to take an estimation of an estimation.

Then I can look at the 2,138. There's not always a perfect answer here. Sometimes you might want to round it to 2,140; sometimes you might want to round it to 2,100. But for my head, it's a lot easier to multiply 2,000 times 15 than 2,100 times 15 or 2,140 times 15.

So that's what I'm going to do. I'm going to 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— 2,000 times 15. And that I can do in my head because 2 times 15 is 30. I'm actually dealing with not two, but 2,000, so it's going to be 30,000.

So roughly, or about, 30,000 people took photos with their phone.