So I have been practicing with the gmatprep and I have consistently scored 51s and some occasional 50s in math section. My impression is that the higher math problems test for your ability to manipulate the stems and rework the equations and most DS questions are number properties questions.
But I wanted to see how the difficult questions relate to the "easy" questions, so I intentionally missed the first 15 questions. To my surprise, the easy questions were actually "hard". I found that they usually ask you to do actual multistep calculations that are time consumming and error prones. I recognized some questions similar to questions that were posted by members who were not familiar with the gmatprep tests, because I thought some of those questions were particularly tricky. Some of questions were asking for estimated values, but the answer choices usually have 2 values very close to each other, so they require actual calculations.
This is giving me some concerns because I might be spending a lot of time on easy questions and I might get them wrong by overlooking obvious clues.