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## 3rd grade

### Course: 3rd grade > Unit 8

Lesson 2: Estimation word problems# 2-step estimation word problems

CCSS.Math:

Sal solves a 2-step estimation word problem with whole numbers.

## Want to join the conversation?

- I'm confused. How does this work?(9 votes)
- In my opinion, it actually doesn't in any practical sense.

The idea is that by rounding the numbers, you get an easier calculation that gives you a rough approximation of the number you're after, but even in this example in which the rounding of the starting numbers is small, we end up with a 10% margin of error, which to me seems unacceptable.

Obviously it depends on your purpose, but like I said, these are very small roundings and yet the margin of error is already quite massive in my opinion. In larger numbers you can imagine the margins growing even more unpredictable and significant.(3 votes)

- I didn't understood :( Why should we change the amount?(3 votes)
- because, the question asked to estimate. So sal rounded to the nearest tens place(16 votes)

- i got 90

28*4=112

112-22=90

90 is in tenths so am i wrong im confused(6 votes)- you are suposed to round 28 and 22(2 votes)

- That doesn't make sense on the multiplcation equation.(4 votes)
- 70?is it 70? I NEED TO KNOW!(2 votes)
- it's 100 because you round 28 to 30 and you don't need to change the four and then you round 22 to 20 and then you do [30 x 4]-20 and then you get 100(1 vote)

- what does estimate mean?(1 vote)
- estimate means you don't have to find the exact number.(3 votes)

- because you are estimateing the amount so you change the number!(2 votes)
- I love Khan Academy(2 votes)
- Do you know Jesus?(2 votes)
- how is it posble(1 vote)

## Video transcript

- [Instructor] We are told
that a race car driver has 28 cars. Each car has four tires. He has to replace all
the tires on the cars. He has 22 tires right now. Estimate the total number
of tires he needs to buy. So pause this video and
see if you can do that. And the key word here is estimate. You don't have to figure out exactly. You just need to figure out roughly the total number of tires he needs to buy. All right, now let's work
through this together. And as I mentioned, the
key word here is estimate. And estimating is something
we'll do throughout our lives. I probably do it once a day at least. And when you're estimating, you want to make all the
numbers roughly right, but also think about,
hey, what do I need to, how do I need to them about them in order to make the
computation a little bit easier or a little bit friendlier? So if I wanted to figure out exactly how many tires he needed to buy, I would say, hey, he has 28 cars. He has 28 cars. Each car has four tires. So each car has four tires. So if you wanted to know the
total of number of tires, you would multiply 28 times four. And then it says he has to replace all the tires on the cars. And if he didn't have any, this is the number that
he would have to replace, but it says he has 22 tires right now. So if we were trying
to figure out exactly, you would say 28 times four, that's how many he needs to replace, but he already has 22,
so you subtract that out, and this whole number would
be the number he needs to buy. Now this calculation isn't so easy to do, especially if you're doing it
on a napkin or in your head. But that's why we're going to estimate. So if we're estimating, we say, hey, what's a friendlier number than 28? Well, 28 is pretty close to 30. So I could say, so if I say,
hey, 28 is pretty close to 30. So that's our way of making
the 28 a little bit friendlier. Four is already pretty easy to deal with. And then the 22, well,
that could be a lot easier to deal with, if instead of it being a 22, if that were, let's say that's
approximately equal to 20. So 22 is approximately, roughly equal, that's what these squiggly
equal signs mean, to 20. And so that simplifies
the calculation a lot. We could say that this thing
up here is roughly equal to, he roughly has 30 cars, and each of those cars need four tires, and then he's going to figure
out the total number of tires he needs to replace, but
then he subtracts out the number that he already has. So instead of subtracting out 22, we're gonna subtract out
roughly how many he already has, roughly 20. And this computation
right here is a lot easier to do in our head. Three times four is 12,
so 30 times four is 120. So this part right over here is 120. And then you subtract 20 from that. Well, 120 minus 20, well,
that is going to be 100. So he roughly needs, if I were
to estimate the total number of tires he needs to buy,
it would be about 100 tires.