I agree, with Jack. I went through the practice problems, but only did well after I built a solid foundation using the MGMAT guides
and gave me more confidence that I had a sense of what the GMAT would test.
I would disagree that you ignore the first 50% of problems, unless you are having no issue. Do a few here or there just to a get a feel for them. I went through them quickly so that I can get a sense of how fast I can get through them without making careless mistakes, so I can get to the harder bins on the adaptive test. Also, I found the "easy" questions can reappear as a hard questions in different forms.
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