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Individual problems are not the issue.
The issue is with staggering too many Q50/Q51s within the same set i.e. the difficulty density.
Treat this as drills, not calibration tests. To solve the entire set within 45 minutes one needS to have an olympiad level quant ability.
In actual GMAT, one should get 3-4 (that too at most) of the so called "hard" problems instead of 10-12 that these sets throw up once a clean sequence of first 7-8 is built up.
Even the first 7-8 has scattered landmines.
So bottomline is, don't look at the score you get from them .
These are an excellent collection of highest level quant problems, that GMAT can realistically throw at you.