Main content

## Get ready for 7th grade

### Course: Get ready for 7th grade > Unit 3

Lesson 5: Analyze relationships between variables# Writing equations for relationships between quantities

CCSS.Math:

Sal shows some examples of how to write equations given a context where two variables relate to each other, and how to use those equations to answer questions about the relationships. Created by Sal Khan.

## Want to join the conversation?

- hi how can two variables relate to each other pls make it easy(13 votes)
- just deal with it(4 votes)

- I'm confused because the video is going too fast.(2 votes)
- Change the playback speed so you will understand better.(6 votes)

- please i don't understand this exercise.(3 votes)
- I have a dumb question, probably.

What is vegetable stock? (3:14)

Thanks for taking the time to read this, really, if you have more important things to do, you can continue, I'm just curious.(2 votes)- Vegetable Stock is a liquid made from the juices of simmered vegetables, it's usually used as a base for different kind of soups.

You can also find stock made from meat.(2 votes)

- this is going to help me on my test so thanks.(2 votes)
- It is i the pringles man! Tremble in my presence!! Hahahahaahahah(2 votes)
- me on the outside =D then me on the inside <=((2 votes)
- why are you gay(1 vote)
- Ngl I’m at a all Bot school and they be gay(2 votes)

## Video transcript

- [Narrator] We're told
Amad is going to walk 20 kilometers for a charity fundraiser. In the first part of
this question, they say, write an equation that
represents how many hours, t, the walk will take if Amad
walks at a constant rate of r kilometers per hour. Pause this video and see if
you could have a go at that. All right, now let's work
through this together. So you might be familiar with the notion that distance is equal to rate times time. For example, if you were to walk at a rate of five kilometers per hour for two hours, you would say five times two,
five kilometers per hour, times two hours, would
give you 10 kilometers. Now in this situation, they've given us the number of kilometers or the distance in this situation, so in this situation d is equal to 20. So 20 is going to be equal to our rate, which we are told is going
to be r kilometers per hour, times our time, which is t hours. Now they're asking us for an equation that represents how many
hours the walk will take if Amad walks at a constant rate of r. So the way that it's phrased, it sounds like they
want us to solve for t, where t is going to be equal
to some expression here that deals with r and
probably some other things, so if we put in any r here,
then we can get the time. So if we know what the rate is, if you put that in here, because it's already solved for t, we'll be able to solve for that time. You could think of r as
the independent variable that you could try
different values out for, and that t is the dependent variable. It's the thing that we have solved for. So let's do that. Let's rewrite this expression
here by solving for t and I could do it right over here. If I have 20 is equal to rt, if I wanna solve for t, what can I do? Well, I could divide both sides by r. If I do that on the right-hand side, then I'm just left with a t here, because an r divided by r is just one. And on the left, I have 20 over r, so I have t is equal to 20 over r and we're done. This will tell us how many hours Amad will take to walk based on the rate. You give me a rate. I'm just gonna divide 20 by that and I'm going to give you t. You might say, why is this useful? Well, this is useful because
now that we have it written this way, any time
someone gives an r to you, you just take 20 divided
by that and it essentially is already gonna solve
for what your time is, how long Amad's gonna have to walk. Question two, how many
hours will the walk take if Amad walks at a constant rate of 6 kilometers per hour? Well, here is an example of that, where they are giving us the actual rate and they want the time. So we just take the 6
and replace it in for r, so we get t is equal to 20 over 6, which is 3 and 1/3 hours, which would be the same thing
as 3 hours and 20 minutes, depending on how you wanna view it. Let's do another example here. So here we're told at the end of each day, a restaurant makes soup
with whatever amount of vegetable stock is unused that day. Let me re-center this a little bit. The soup recipe calls for
400 milliliters of water for every 500 milliliters
of vegetable stock. Write an equation that
represents how much water the restaurant should use,
and we'll use the variable w, with any amount of vegetable stock, v. All right, and then we'll do
part two right after that, so let's look at this,
400 milliliters of water for every 500 milliliters
of vegetable stock. And so to get my head around this, I like just to think about, let's put a little table here, and so you could say amount of water, let me write it this way, water and vegetable stock, vegetable stock. So for every 500 milliliters
of vegetable stock, so if you had 500 milliliters
of vegetable stock, and I won't write the milliliters, then you're going to have
400 milliliters of water. If you had 1,000 here,
which is two times that, well, you're gonna have
twice as much water, which is going to be 800. And so you can see this
relationship that's forming. No matter what the vegetable stock is, if you essentially take 4/5 of that, that is the amount of water. You take 4/5, that is the amount of water. So if you had only 5
milliliters of vegetable stock, you take 4/5, you get the amount of water. So another way to think
about it is the water that you need to use is going to be 4/5 of the amount of vegetable stock that you are going to be using, and so actually we just did part one. We wrote an equation that
represents how much water the restaurant should use with
any amount of vegetable stock and the way that they phrased it, we're solving for w given
some v that you might have. And since we're solving for w here, we would consider w the dependent variable and v as the independent variable. You can give me different vs and then I can put that into
this little equation here, and I can solve for the w, so we've done the first part. If there are 800 milliliters
of unused vegetable stock, how much water should the
restaurant use to make soup? Well, we can just take this 800 and substitute it in for
v to figure that out. In this situation, the
amount of water to use is 4/5 times 800, and that's going to be,
let's see, 800 divided by 5, 100 divided by 5 is 20 and so, and then you're gonna have 8 of those, so it's 160, so this was 160 times 4 is equal to 640 milliliters, and we are done.