I've completed the next 40 questions set from quantitative GMAT review book. Well, these were almost flawless, but with a few careless errors. If I would actually write the equation down, I'd get them right, but trying to keep everything in head doesn't help accuracy. So far 80 questions out of 176 in the Problem Solving section and I'm quite disappointed. Well, there are too easy. I don't even think these are 600-level ones. However, the lesson that I learnt is that no matter how easy is the question, you can make a mistake there because of being careless.