GMAT Reading Comprehension (RC) | Strategies for the 6 Reading Comprehension Question Types

Himargarette ,

What is difference between main idea and purpose of passage?

I try to answer below mentioned questions to get main idea andpurpose...
Main idea : Why am I reading this passage? / What usefullness does the passage serve?
Purpose : Why author wrote the passage?

Is my understanding correct?

Also, in question typeAccording to passage , should the correct answer be given as is in the passage or we might have to make certain inferences?
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