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Rule of three - Conceptual question

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Rule of three - Conceptual question

by metallicafan » Mon Dec 17, 2012 7:46 am
I know that I have to use the "rule of three" when the relationship between the variables is direct and proportional. Identifying that the relationship is direct is easy; however, I don't know how to be sure that that the relationship is proportional. Is there a method or tecnique to confirm that?

For example,
If 7 candies represent the 35% of a bag of candies, how many candies are in the bag?
Common sense tells me that I have to use the rule of three:

7 ----- 35
x ----- 100

x = 20

Although that's the correct answer I was not sure that the relationship is proportional. How to confirm that in this problem and in more complex questions?
Thanks!
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by Jim@StratusPrep » Mon Dec 17, 2012 2:38 pm
you can just treat this like an equation and solve for x no matter how hard the math becomes.
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by ceilidh.erickson » Sat Dec 22, 2012 2:54 pm
To answer your question in a more conceptually abstract way... percents are always proportional information. If you're saying that 7 is 35% of something, you're really saying that the relationship of 7 to the total is the same as the relationship of 35 to 100. In other words, 7/x = 35/100. Or you could compare in the other direction - the relationship of 7 to 35 is the same as the relationship of the total to 100: 7/35 = x/100. Once you cross-multiply, you see that the math is identical.

Did this answer your question about how to figure out if something is a proportion?
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EdM in Mind, Brain, and Education
Harvard Graduate School of Education
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