You've done the first step right by acknowledging the weak areas.
Now, take the questions that don't make sense and classify them as to why they don't make sense.
For many questions, you may slap your forehead when you see how easy it really is.
Remember that the GMAT uses twisted language to intentionally obscure the fundamental concepts of what it is asking, especially in the Math section.
1) rewrite each question in plain English. It often helps to write it out longhand.
2) If you still encounter a fundamental misunderstanding in the math itself, I suggest you find a tutorial online that covers that particular topic but doesn't present it in "GMAT style."
3) Take the question and type a pertinent section of it into Google, within quotation marks. Likely someone has discussed it on one of these forums. Perhaps an alternative explanation will make more sense.
4) If you're working from a book that has explanations, feel free to modify the explanations as you see fit. Rewriting the explanations will help classify and organize your thought--immensely. The OG explanations may be "simple" for the Mathematician/Statistician who wrote them, but they are not necessarily the most convenient way to solve a problem if you're under time constraints.
5) I know this seems like a lot of effort, but you will be shocked how much better you begin to see things after deconstructing, say, 10 questions. The same process is perfectly applicable in Verbal as well.
It is a useful tool in any case, but its impact is felt only by those who take the time to use it!
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I am Rowan Hand, a London GMAT Tutor, Consultant, and Coach. Based in the UK--available anywhere!