I have read many RC primary purpose / main point questions and found that there is a trick about these kind of questions. I am sharing my experience hope to have your ideas about it.

Actually almost all of RC passages have one primary purpose question. If you look at these questions within the OG/PREP passages and compare them with each other you can find that most of them follow this pattern:

First check the answer choices and find out the key element of each choice. Consider these words which i have taken from PP questions of the passages:

rc-naked-mole-rates-let-s-discuss-the-best-rc-strategies-32552.html

*state a conclusion /introduce information/qualify the extent / show the chain / demonstrate that

http://www.litesee.com/verbal/verbal/re ... sage56.htm
* argue against/ compare and contrast / provide example /suggests that theories /summarize a number of

prior-to-1965-geologists-assumed-that-the-two-giant-rock-32112.html
* evaluating a method /discussing explanations /examining the assumptions /questioning the validity /presenting evidence

manufacturers-have-to-do-more-than-build-large-manufacturing-133422.html
* point out the importance /show that/ argue that / suggest that / explain why  

So, this is my solution for PP questions:

1.After reading the passage. First take a look at key words of each PP answer choice. this may take 10 sec.

2. think that why the author has written the passage?
you can make questions with keywords such as:
Is he discussed something? has he made an argument? Does he has a suggestion? Is he evaluated some theory? ...

3. easily you can eliminate some choices and if there are 2 options then read the whole statement of answer choice. (10 sec)

4. Finally , before selecting the right answer choice you must read the whole statement avoiding to fall in the traps of the choices as GMAT likes to trap you.
(10 sec)

I have practiced this method through many RC passages of OG 10,11,12,13 , V2.0 and PREP. In 90% of questions it works for me . In other 10% I had vocab problems with keywords!.
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