To chime in, yes, practice reading denser material. You'll become good at discerning what main ideas are and how arguments are constructed. Another tip is to learn how to take notes on RC passages. If you can manage scribble down main ideas and supporting arguments while you area reading / scanning the passage, you will not need to go back and re-read the passage when you answer each question.
Since more than 95% of all correct answers for Main Idea and Structure questions come from the first two sentences of the paragraphs, your first step is to read the first two sentences of each paragraph and take notes on each.
Make sure you only paraphrase and do not copy everything word-for-word. The point is is to get a brief summary of what you have read.
Pay attention to any of sentences that have opposition conjunctions like however, but and although.
This should be enough for a general mapping of the text and should help you locate many of the detail-related questions.
Before you get the hang of note-taking, don’t worry about timing. Your timing will improve as you practice. Most people complain about the fact that note-taking takes more time, but in the end, it will actually save you time and make you more precise.
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