GMAT doesn't ask very difficult questions in Quant, although you might want to practice those for enhancing your prowess. Then again, as per my experience/observation, every book gives a mixture of questions of various difficulty levels. Perhaps you can keep the severly tough problems aside for now, and return to attack them once you score 49+ consistently. The other thing is, my opinion is slightly subjective as I have not seen the book/contents.
Formula of Life -> Achievement/Potential = k * Happiness (where k is a constant)
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