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## Algebra 1

### Course: Algebra 1>Unit 3

Lesson 2: Appropriate units

# Defining appropriate quantities for modeling

Examples for different ways to define how quantities of interest are calculated in a modeling problem.

## Want to join the conversation?

• this still confusing and I STILL NEED HELP PLEASE
• If you can identify the outputs and inputs and if you can do divisions you’re done. You just have get good at identifying input and outputs. Then divide input by output
• I always get the second question wrong.
How do i know if they're the same or they're opposite? It's really confusing.
Thanks :)
• Try extending the table on your own by just multiplying and dividing a bunch of values. It saves you time and tedium, and isn’t that much work because there are only two different tables.
• I would suggest that people just skip over this lesson. It makes no sense. Even when I looked at the answers review its telling me to divide 900/190/3000 to get an answer but the only way I could get the answer it produced was to multiply at the end and not divide. Ive been stuck on this lesson for about a week and it still makes 0 sense and its obvious that it has to do with the explanation itself. A lot of the lessons give you a simple problem in the videos and then when it comes to doing the practice tests or lesson tests there are questions and things that were not gone over. Which is why most people are finding the curriculum a bit confusing. Mainly because there are key important information's not being given.
• i would probably skip this but I can't stand it saying something like 99% Mastery
• man I did some of the equations ahead and I am not getting any of it. hope ya'll are doing better
• I understand.
Sal usually does a great job but he didn't explain this very well.

When you calculate you definitions it can be confusing to find if one produces (for example) more content per writer.
Let show an example:
The question is to find which website produces more content per writer.
You find your definitions on how to figure it first.
1) Posts divided by writers
2) Words divided per writer
(This is after calculation)

Website A:
22 posts per writer
And 2,200 words per writer
(100 words per post divided by number of posts (22) )
That is Website A's content per writer.

Website B:
18 posts per writer
3,060 words per writer
That is Website B's content per writer.

At first glance you'd probably like to say that B has more content, right?
But you must compare both A and B's posts and words per writer.
Posts per writer: [ A = 22 ~ B = 18 ]

Words per writer: [ A = 2,200 ~ B = 3,060 ]
'B' has more words per writer.

This means they have opposite results.
Why?
But 'B' has more words per writer. So they have opposite results.

Now, if they both had 22 posts per W but the words per W stayed the same, only then you could conclude that B had more content.
Does that help or make any sense? I hope so :)
• Please explain why we call this idea
"Defining Appropriate Quantities for Modeling".
What specifically does that mean? Where are the models? Why do we say "define" instead of find? Why isn't this subject named "How to analyze & compare information from tables"? I understand why they put "appropriate", though. I think it was because in these problems we always have to decide which data is more relevant.
• how would you know what is an input or an output?
• It is generally agreed that an input is what you are putting into the thing that you are creating, e.g kilograms of metal bought, and an output is what you get because of it, e.g profit from cars sold.

Hope this helped!
• i am coming back to this video after skipping it and the following exersize about a month ago, not even my very smart parents could do it. i just want to ask this question: how do you DO the exersize!
• This one is definitely tricky, but it's less the concept being difficult and more the many bits of information you have to keep organized.

First, you are going to figure out the quantities you need to compare. What is important? Do you need total teachers or teachers per student? Put the quantities you need on a chart: One side for "Whatever A" and "Whatever B". Make sure the two question for each category are lined up. Then calculate them all.

Next, you are going to check which values are bigger. Is website A's revenue divided by posts larger than website A's revenue/posts?
And is website A's revenue/writers also bigger than website B's revenue/writers?

If one has both of its respective values larger than the others, then the info is consistent and you can say "Thing A/THing B has more" [sports achievement/revenue/productivity/whatever]. But if the values don't line up (say, website A generates more revenue per post but less revenue per word), then the calculations have opposite results.

That was a lot, but I hope it helped! I definitely struggled with keeping the information consistent on this exercise at first, but hopefully you also will get better once you can keep it all straight. :)
• For the first question, why can't it be area/volume instead of volume/area?
• It seems both are technically correct and that it is a matter of convention. You are forced to use your unit together with other established units, so if the other units use this kind of convention then if you use area/volume you wouldn't be able to simplify it with the other units. In real world practice I think you should use whichever helps you figure out and solve the problem at hand the easiest. There seems to be a subtle difference between fractions and units, which has to deal with conversion of units, since 1/10 =/= 10/1, but, 1 meter / 10 seconds = 10 seconds / 1 meter, so, that is something to think about.
• If your confused, don't feel alone, because literally everyone who commented here is also confused.
• yeah but its really getting on my nerves. He isnt explaining it properly. Its super annoying