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### Course: Algebra basics>Unit 3

Lesson 7: Writing & solving proportions

A recipe for oatmeal cookies calls for 2 cups of flour for every 3 cups of oatmeal. How much flour is needed for a big batch of cookies that uses 9 cups of oatmeal? Created by Sal Khan and Monterey Institute for Technology and Education.

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• This just confused me, and I tried to do the problems for the Proportions 1 thing, Im confused now, because these are to difficult to me... help please...
• 9 years and nobody responded
• wow that's relatable
• About proportions...some definitions say "one variable related to another variable by a constant ratio". But the proportionality test
says x/(f(x))=k, where k is constant. So any equation of a line would work, as long as the line intersects (0,0)? In physics,
we are taught that current, voltage, and resistance are proportional to each other in a simple circuit - current is zero when voltage is zero. But what about Farenheit to Celcius conversion? Isn't C proportional to F? Mathematically speaking,
I would say no, but logically I would say yes...apparently proportionality gets used (wrongly) interchangeably with linearity.? In the C F problem, what is the proper term for the C F relationship?
• I don't know much about physics, but I can teach you what proportions are in math. So you mentioned x/(f(x))=k? Close. It's y/x=k. y can also be f(x) though. A proportion is basically a relationship between two numbers that is always constant. If you were to graph a proportion, it would be a straight line that passes through the origin (0,0). In the F and C question you had, the relationship would be F=1.8C+32, or f(C)=1.8C+32. It is not proportional because it doesn't pass through the origin. It instead has a y-intercept of (0,32). An equation can be a linear equation, but it may not always be proportional.

Hope this helps. :)
• anyone else just scrolling the comments cause your teacher blocks everything else?
• so real
(1 vote)
• Could i get some help with proportion problems?
• You could do cross-multiplication to solve proportions, but remember that the unknown value is always the denominator in the answer. It works because you want to isolate the variable, so you could multiply the side with the variable by the denominator and then do that with the other side. Then simplify the fraction.
• I knew the answer before the first 30 secs of the vid
• How about if you have a problem like this?
Jim uses 3 cups of peaches to yield 4 jars of peach jam. He also makes strawberry-peach jam. He uses equal amounts of strawberries and peaches. How many cups of strawberries does Jim need to yield 10 jars of strawberry-peach jam? If someone could walk me through how to do the problem and why that would be greatly appreciated! Thanks in advance! :)
• thanks sal but what if i do it this way 2/3.9/1 and then simplify it and still get 6 is it correct
• Hey Bryan A. I feel bad for you. You have waited 11 years and your question still hasn't been answered
• I still want my mcdonalds cookies😭
• Let me just share here how important Math is even when you are hating it cos a lot of things don't come naturally and perfectly alright to feel that way. 99% struggle with Math initially- Very few are born geniuses to whom Math comes naturally. It is really important to spend more time, rewatch these amazing tutorials to get a complete grasp. Remember, everything is interrelated in Math so it is crucial you get all the basics right or else you will always feel lost. I'm 30 years old and I'm back here to refresh my memory on math so I can tutor my kids better. Math is everywhere- Computers, Business, etc. So just keep at it. Don't give up and the efforts you put here will reap tremendous benefits in the future. Also, feel free to explore more youtubers on Youtube as sometimes things clicks better when we try different ways.

Nonetheless, the best way is to simply play around with the problem.

For eg- the concept of an equation is a marvellous thing.

We can start with 1=1 and to keep it that way we need to do on the right what we do to the left and vice versa because 1=1 by definiton means they are equal.

so if I do 1*5 = 1, the equation becomes invalid cos the lest is 5 and the right is still 1- so we do 1*5 = 1*5. We basically keep doing the same thing even when thigns get more abstract. So just playing with the basic equation and trying to manipulate it in as many ways as possible builds a good intuition to solving these problems which are super important in day to day life.