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Struggling with ratio DS questions

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Senior Manager
Joined: 08 Dec 2015
Posts: 285
GMAT 1: 600 Q44 V27
Struggling with ratio DS questions  [#permalink]

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03 Jan 2017, 10:09
Hi everyone!

I am having a huge problem with DS ratio questions. I can get most of PS right, but I get 90% of DS wrong.

Do you know of any page that would teach me the theory behind these questions? Like the quarter wit from @Karishima for example.

Thank you!
Magoosh GMAT Instructor
Joined: 28 Dec 2011
Posts: 4473
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03 Jan 2017, 12:58
iliavko wrote:
Hi everyone!

I am having a huge problem with DS ratio questions. I can get most of PS right, but I get 90% of DS wrong.

Do you know of any page that would teach me the theory behind these questions? Like the quarter wit from @Karishima for example.

Thank you!

Dear iliavko,

I'm happy to respond. First of all, here's a blog you might find helpful.
GMAT Quantitative: Ratio and Proportions

I will say that all math happens in the details. You asked a very general question here. I will say some general things in response, but it may be that to resolve your confusion, you will have to post some individual questions with which you struggle.

One BIG idea #1 is that there are two kinds of information in such problems:
1) Ratio information = this includes not only the ratios but any fractional relationships and percent increases/decreases
2) Count information = the exact number of individuals

Big idea #2 is that if we have only ratio information, no matter how much we have, we can never generate count information. For example,
Prompt: In a class, the ratio of boys to girls is 2:3. How many boy students are there?
Statement #1: If the number increased by 50%, then there would be the same number of boys and girls.
In this concocted problem, the prompt gives us ratio information and ask for count information. Statement #1 give us more ratio information. If all we have is ratio information, we can find a count. Statement #1 is insufficient.

Big idea #3 is that a ratio combined with ANY single piece count information is enough to determine all the counts. If the ratio is A:B:C:D:E, then not only the count of any one of those items, but the count of any sum or difference (e.g. B has 16 more member than C does), would qualify as the single piece of count information needed to find all the other count information.

Big idea #4 is that we always can turn ratios into algebra: if the ratio of A:B:C is 4:7:9, then these groups are, respectively, 4n, 7n, and 9n, for some integer n.

I don't know whether this answers your questions. Please let me know if you have further questions.
Mike
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Mike McGarry
Magoosh Test Prep

Education is not the filling of a pail, but the lighting of a fire. — William Butler Yeats (1865 – 1939)
Senior Manager
Joined: 08 Dec 2015
Posts: 285
GMAT 1: 600 Q44 V27
Re: Struggling with ratio DS questions  [#permalink]

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07 Jan 2017, 11:47
Hi mikemcgarry

You always have interesting info to teach

I will practice some more so I can come up with more specific problems I have.

Thank you!
Manager
Joined: 25 Jun 2016
Posts: 61
GMAT 1: 780 Q51 V46
Re: Struggling with ratio DS questions  [#permalink]

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07 Jan 2017, 15:47
1
Many of my students have benefited greatly from what I call the "Chart Method for Ratios," especially in Data Sufficiency.

In the following playlist, I introduce the chart method, show how it's particularly useful in DS, and then solve a few questions from the Official Guide:

Senior Manager
Joined: 08 Dec 2015
Posts: 285
GMAT 1: 600 Q44 V27
Re: Struggling with ratio DS questions  [#permalink]

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10 Jan 2017, 17:29

Thank you a lot for the video! It develops really well on the ideas brought by Mike.

You see it's frustrating because I understand the concept quite well (I think) but then I get really confusing questions such as the one by Magoosh that says something like: a conference had members and guests and a member could only bring one guest, etc. mikemcgarry Do you see which one it is? Like it's really confusing and I don't see how applying these concepts helps. It seems that the Q is testing something else..
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Re: Struggling with ratio DS questions  [#permalink]

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18 Dec 2018, 15:47
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Re: Struggling with ratio DS questions   [#permalink] 18 Dec 2018, 15:47
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