If you want advice from other students, you might want to post this question in another thread; this thread is specifically for me (a ManhattanGMAT employee) to answer questions.
That said, I do not consider most challenge problem questions to be indicative of the official GMAT. At least half of them are harder than anything you can expect to see on the test. Of the rest, they are all at least 700-level questions, most more like 750+, and you'd be unlikely to see more than a few of these on the official test.
If you're really looking for a stratospheric score, then you may want to expose yourself to these so that the actual test feels a little easier. Or if you already have a degree in math or engineering, then you may want to study from these just to challenge yourself a bit. But they are only an appropriate study tool for a very small minority of testers.
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Stacey Koprince
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Manhattan GMAT
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