The GMAT is a computer-based, adaptive exam; student needs to take this into account while practicing. The exam literally judges you on every question. The algorithm of this adaptive exam is a tough nut to crack. If you get the first question right, then the next one, presumably, will be higher level question, and if you get a question wrong, then the next one will be lower level one. Also, the strategy that Indian students mostly employ in other competitive exams, such as CAT, NMAT, XAT, etc to leave/ skip a question and come back to it later is a luxury that you cannot afford on GMAT. You have to compulsory mark a question before moving on. It is imperative that you finish the Quant or Verbal section on time, otherwise a penalty is levied that has the potential to suck 60-70 points off your total score, depending on the number of questions you leave unattempted.