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 ProportionsI 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

_________________

Mike McGarry

Magoosh Test Prep

Education is not the filling of a pail, but the lighting of a fire. — William Butler Yeats (1865 – 1939)