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## Algebra basics

### Course: Algebra basics>Unit 3

Lesson 7: Writing & solving proportions

# Proportion word problem: cookies

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.

## Want to join the conversation?

• 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
• 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. :)
• I should probably pay attention instead of scrolling comments lol
• anyone else just scrolling the comments cause your teacher blocks everything else?
• 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! :)
• Better Call Sal
• 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.