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## 8th grade (Eureka Math/EngageNY)

### Course: 8th grade (Eureka Math/EngageNY) > Unit 4

Lesson 4: Topic D: Systems of linear equations and their solutions- Systems of equations: trolls, tolls (1 of 2)
- Systems of equations: trolls, tolls (2 of 2)
- Testing a solution to a system of equations
- Solutions of systems of equations
- Systems of equations with graphing
- Systems of equations with graphing
- Systems of equations with graphing: 5x+3y=7 & 3x-2y=8
- Systems of equations with graphing: y=7/5x-5 & y=3/5x-1
- Systems of equations with graphing: chores
- Systems of equations with graphing
- Systems of equations with elimination: 3t+4g=6 & -6t+g=6
- Systems of equations with elimination
- Systems of equations with elimination: x+2y=6 & 4x-2y=14
- Systems of equations with elimination: -3y+4x=11 & y+2x=13
- Systems of equations with elimination: 2x-y=14 & -6x+3y=-42
- Systems of equations with elimination: 4x-2y=5 & 2x-y=2.5
- Systems of equations with elimination: x-4y=-18 & -x+3y=11
- Systems of equations with elimination
- Systems of equations with elimination: 6x-6y=-24 & -5x-5y=-60
- Systems of equations with elimination challenge
- Systems of equations with substitution: 2y=x+7 & x=y-4
- Systems of equations with substitution
- Systems of equations with substitution: y=4x-17.5 & y+2x=6.5
- Systems of equations with substitution: -3x-4y=-2 & y=2x-5
- Systems of equations with substitution: 9x+3y=15 & y-x=5
- Systems of equations with substitution
- Systems of equations with substitution: y=-5x+8 & 10x+2y=-2
- Systems of equations with substitution: y=-1/4x+100 & y=-1/4x+120
- Systems of equations with elimination: apples and oranges
- Systems of equations with elimination: TV & DVD
- Systems of equations with elimination: King's cupcakes
- Systems of equations with elimination: Sum/difference of numbers
- Systems of equations with elimination: potato chips
- Systems of equations with elimination: coffee and croissants
- Systems of equations with substitution: coins
- Systems of equations with substitution: potato chips
- Systems of equations with substitution: shelves
- Systems of equations word problems
- Age word problem: Imran
- Age word problem: Ben & William
- Age word problem: Arman & Diya
- Age word problems
- Solutions to systems of equations: consistent vs. inconsistent
- Systems of equations number of solutions: fruit prices (1 of 2)
- Systems of equations number of solutions: fruit prices (2 of 2)
- Systems of equations number of solutions: y=3x+1 & 2y+4=6x
- Solutions to systems of equations: dependent vs. independent
- Number of solutions to a system of equations graphically
- Number of solutions to a system of equations graphically
- Forming systems of equations with different numbers of solutions
- Number of solutions to a system of equations algebraically
- Comparing Celsius and Fahrenheit temperature scales
- Converting Fahrenheit to Celsius

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# Systems of equations with elimination: coffee and croissants

Sal solves a word problem about the price of coffee and croissant by creating a system of equations and solving it. Created by Sal Khan.

## Want to join the conversation?

- Couldn't you just look at it and say: 5.3 * 2 is not equal to 14!(27 votes)
- Of course you could, but that wouldn't be using a system of equation... I have travelled to Paris and this will happen to you.(12 votes)

- I've mastered this skill before and watched each video like 5,000,000 times ,but I got it wrong on a mastery and now i can't seem to get ANY of the questions right. Does anyone have any tips ?(21 votes)
- THIS IS GOING TO BE LONG..... I'll vote up your question.

So, before I get all scientific. That happened to me too! So, you mastered tis skill already, and watched it like 5,000,000 times, right. Well, what happens in your brain is all that information is getting stored. When, you watched it a lot and mastered the skill, the brain says " Okay, empinda knows this." And stops receiving information, and the the other things that you learned are disappearing, because you are practicing it too much. Now, that doesn't work for everything you study., What do you need too do in order to walk? Well, you have to know how to stand up, and move. That is simple so the brain does not forget about it, even if you practice it 5,00,000 times, you do it every single day, so the brain knows you need this, but solving equations you have to set up the equation, cancel it out, plug it in, and solve, etc. so, everytime you solve equation, your brain goes though all that, and soon your brain can't take it so, it disappears. And you probably learned more stuff after you watched it 5,000,000 times so as new stuff come in old stuff go out. Simple stuff, or important stuff don't go out.

Tip: don't study it that much, or find out a easier way!(27 votes)

- I’m sad you spent so much time on these really easy problems and hardly any time on the very difficult distance/ & rate problems. I struggle with distance and rate problems but you have many more examples of the much simpler problems. Why is that?(13 votes)
- How did 14 become 10.60?(5 votes)
- 14 didn't become 10,60.

We're comparing two situations with the same variable so it can be a little bit tricky but if it can help you :

Cup of coffee bought by you : X1

Croissant bought by you : Y1

Cup of coffee bought by the local : X2

Croisant bought by the local: Y2

You know that X2 + Y2 = 5,30

and : 2x1 + 2y1 = 14

You want to know if you paid the same price as the local. To verify this, X1 should = x2 and y1 should = y2, or in the video we say 2x1 = 2x2 and 2y1 = 2y2.

We then put two DIFFERENT equation to see if we can conclude something about it.(6 votes)

- Can I call this system of equations "inconsistent" ?

Thanks in advance.(4 votes)- Yes, this system of equations is inconsistent. The slope of both lines is -1. But the y-intercepts are different. When graphed this will produce two parallel lines. Since parallel lines will never intersect, we call this an inconsistent system.(7 votes)

- In the problem I think it should be worded differently. Like I paused the video and tried it and I said no not possible cause it seems like that it says one coffee and one croissant for 5.3 euros. Not total of 5.3 euros. Or am I reading this wrong?(5 votes)
- So how did you interpret the statement "a cup or coffee and a croissant for 5.3 euros"? I would have seen this and immediately said let f be coffee and c be croissant, so 1f + 1c = 5.3. And often is an addition word in Math. Yes total would have been clearer but I cannot figure out any alternate interpretation.(2 votes)

- why wouldnt u just ask how much it costed(4 votes)
- Because then it wouldn't be a math problem.(1 vote)

- this is so easy problem no need to solve. just a trick is needed. you just look at it and say (5.3) * 2 is not equal to 14.(4 votes)
- Why does Sal write 'let' before he defines his variables?(2 votes)
- Because we are using a variable, not a constant and a variable is not a fixed term for the given situation and everyone can choose their own variables.(2 votes)

- You have used substitution and elimination methods to solve the pairs of linear equations but what about the cross-multiplication method? How will I use that?(2 votes)

## Video transcript

You are at a Parisian
cafe with a friend. A local in front of you buys a
cup of coffee and a croissant for $5 or 5.30 Euro. When you and your friend get
two cups of coffee and two croissants, you are
charged 14 Euro. Can we solve for the price of
a cup of coffee and croissant using the information
into a system of linear equations
in two variables? If yes, what is the solution? If no, what is the
reason we cannot? So we're looking
for two things-- the price of a cup of coffee
and the price of a croissant. So let's define
two variables here. Since we have all
these C's here, I'm just going to
use x's and y's. So let's let x be equal to the
price of the cup of coffee. And let's let y be equal to
the price of a croissant. So we first have
this information of what the local
in front of us did. The local in front of us buys
one cup of coffee and one croissant for 5.30 Euro. So how would we set
that up as an equation? Well, we got one cup of coffee. So that's going to be one
x, or we could just write x, plus one y because
he got one croissant, and it cost 5.30 Euro. So this equation describes
what happened to the local-- bought one cup of coffee, one
croissant, paid 5.30 Euro. Now, when you and
your friend get two cups of coffee
and two croissants, you are charged 14 Euro. So what's an equation
to describe this? So we should be charged
two times the price of a cup of coffee. So it should be 2x. And then we should be
charged two times the price of a croissant, so plus 2y. And the sum of these
should be the total amount that we're charged. So we've been charged 14 Euro. So let's see if we can solve
this system of equations. And there's many, many,
many ways to solve this. But the most
obvious way at least looking at this right over
here is you have x, we have 2x, we have y, we have 2y. Let's take this first
equation that described local and multiply it by two. So let's just
multiply it by two. So we're going to
multiply both sides, otherwise equality
won't hold anymore. So we would get 2x plus 2y is
equal to 2 times 5.30 is 10 euro 60. Now, something very
interesting is going on here. If the local had bought twice
as many cups of coffee and twice as many croissants, he
would have paid 10.60. And that would have been
the exact amount of coffee and croissants you
got and you paid 14. So it looks pretty
clear that you got charged a different amount. You got the tourist rate for
the cup of coffee and croissant, while he got the local rate. And we can verify that
there's no x and y that's going to satisfy this. And even logically
it makes sense here. 2x plus 2y is 14. Here, 2x plus 2y is 10 euro 60. And we could even show
that mathematically this doesn't make sense. So if we were to subtract this
bottom equation from this top, so essentially you could
imagine multiplying the entire bottom
equation times negative 1. So let's multiply the entire
bottom equation by negative 1 and then we add
these two equations. Remember all we're
doing is we're starting with say this
equation, and we're adding the same
thing to both sides. We're going to add
this to this side. And we already know that
negative 10.60 is the same thing as this, we're
going to add to that side. So on the left hand side,
this cancels with this, this cancels with this,
we're left with 0. And on the right hand side,
14 minus 10.60 will get you to 3.40. And there's no x and y that
you can think of that can all of a sudden make 0 equal 3.40. So there is no solution. And the only
explanation over here is that the local was
charged a cheaper rate.