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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.

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  • blobby green style avatar for user John Studer
    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?
    (8 votes)
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    • piceratops ultimate style avatar for user jiantw
      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. :)
      (12 votes)
  • hopper jumping style avatar for user Komal
    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! :)
    (6 votes)
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  • blobby green style avatar for user kristinmorgan76
    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...
    (5 votes)
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  • leaf blue style avatar for user 201125079
    Could i get some help with proportion problems?
    (4 votes)
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    • purple pi teal style avatar for user keithcyu
      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.
      (5 votes)
  • leafers seedling style avatar for user LükeMathLearner
    So really in he just listed out what was in the problem straight forward. Right?
    (4 votes)
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  • starky sapling style avatar for user Myla*Mae🍂
    This is abby every time u vote gets older 1 age how old is she?
    😁
    👗
    (4 votes)
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  • cacteye purple style avatar for user mickatia
    so i have this math problem



    five pounds of apples cost $3.70.
    a.how many pounds could you buy for $5.91?
    (1 vote)
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    • piceratops ultimate style avatar for user Hecretary Bird
      For this problem, you can set up a proportion, as the cost per pound of apples stays the same no matter if you by 5 pounds of however many. When setting one of these up, just make sure that the units match. The prices should both either be in the numerator or denominator, and the apples should be in the other location:
      5 / 3.70 = x / 5.91
      From here, you just simplify the equation as normal:
      5.91 ( 5 / 3.70 ) = x
      x = 7.98
      You could buy about 8 pounds.
      (4 votes)
  • duskpin sapling style avatar for user Louis28
    I should probably pay attention instead of scrolling comments lol
    (2 votes)
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  • starky seedling style avatar for user daniel.issac.lezcano
    Wait, im confused...
    in this video Sal explained that the problem should be written like 2/3 = 6/9
    However they explained this to me as
    2 3
    /=/ (or like 2/x= 39)
    x 9
    (2 votes)
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  • stelly orange style avatar for user dubya
    anyone else just scrolling the comments cause your teacher blocks everything else?
    (2 votes)
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Video transcript

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? So let's think about what they're saying. They're saying 2 cups of flour. So 2 cups of flour for every 3 cups of oatmeal. And so they're saying, how much flour is needed for a big batch of cookies that uses 9 cups of oatmeal? Now we're going to go to a situation where we are using 9 cups of oatmeal. Let me write it this way-- 9 cups of oatmeal. And I'll show you a couple of different ways to think about it. And whatever works for you, that works. So one way to think about it, so we're wondering. We're going to say, look, we know if we have 3 cups of oatmeal, we should use 2 cups of flour. But what we don't know is if we have 9 cups of oatmeal, how many cups of flour do we use? That's what they're asking us. But if we're going from 3 cups of oatmeal to 9 cups of oatmeal, how much more oatmeal are we using? Well, we're using three times more oatmeal, Right? We're multiplying by 3. 3 cups of oatmeal and 9 cups of oatmeal, we're using 3 times the oatmeal. Well, if we want to use flour in the same proportion, we have to use 3 times the flour. So then we're also going to multiply the flour times 3. We're going to multiply the flour times 3, so we're going to have to use 6 cups of flour. Ignore that question mark. And that answers the question. That's how much flour we need for a big batch of cookies that uses 9 cups of oatmeal. The other thing is you could set up a proportion. You could say 2 cups of flour over 3 cups of oatmeal is equal to question mark. And instead of writing question mark, I'll put a variable in there. Actually, let me put a question mark there just so you really understand it is equal to a question mark in a box number cups of flour over 9 cups of oatmeal. And so I like this first way we did it because it's really just common sense. If we're tripling the oatmeal, then we're going to have to triple the flour to make the recipe in the same proportion. Another way, once you set up an equation like this, is actually to do a little bit of algebra. Some people might call it cross-multiplying, but that cross-multiplying is still using a little bit of algebra. And I'll show you why they're really the same thing. In cross-multiplication, whenever you have a proportion set up like this, people will multiply the diagonals. So when you use cross-multiplication, you'll say that 2 times 9 must be equal to question mark times 3, must be equal to whatever is in this question mark, the number of cups of flour times 3. Or we get 18 is equal to whatever our question mark was times 3. So the number of cups of flour we need to use times 3 needs to be equal to 18. What times 3 is equal 18? You might be able to do that in your head. That is 6. Or you could divide both sides by 3, and you will get 6. So we get question mark in a box needs to be equal to 6 cups of flour. Same answer we got through kind of common sense. Now, you might be wondering, hey, this cross-multiplying doesn't make any intuitive sense. Why does that work? If I have something set up like this proportion set up, why does it work that if I take the denominator here and multiply it by the numerator there that that needs to be equal to the numerator here times the denominator there? And that comes from straight up algebra. And to do that, I'm just going to rewrite this part as x just to simplify the writing a little bit. So we have 2/3 is equal to-- instead of that question mark, I'll write x over 9. And in algebra, all you're saying is that this quantity over here is equal to this quantity over here. So if you do anything to what's on the left, if you want it to still be equal, if the thing on the right still needs to be equal, you have to do the same thing to it. Now, what we want to do is we want to simplify this so all we have on the right-hand side is an x. So what can we multiply this by so that we're just left with an x? So that we've solved for x? Well, if we multiply this times 9, the 9's are going to cancel out. So let's multiply the right by 9. But of course, if we multiply the right by 9, we have to still multiply the left by 9. Otherwise they still wouldn't be equal. If they were equal before being multiplied by 9, for them to still be equal, you have to multiply 9 times both sides. On the right-hand side, the 9's cancel out, so you're just left with an x. On the left-hand side, you have 9 times 2/3, or 9/1 times 2/3. Or this is equal to 18/3. And we know that 18/3 is the same thing as 6. So these are all legitimate ways to do it. I wanted you to understand that what I'm doing right here is algebra. That's actually the reasoning why cross-multiplication works. But for a really simple problem like this, you could really just use common sense. If you're increasing the cups of oatmeal by a factor of 3, then increase the cups of flour by a factor of 3.