sonigmat, the difficulty of a question is very subjective. You may simply have gotten the type of questions that you didn't really know how to do in the first place on GMATPrep. I think most people here would agree that MGMAT's quant section is a bit harder than the actual GMAT, or GMATPrep. Of course, again, due to the person's own strengths and weaknesses, the difficulty of the question is hard to judge by two different people. The MGMAT just simply seems harder and more process-involved than GMATPrep.
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