If you still got a 640 after guessing on the last 15 Qs, then if you fix the timing problems, I'd say you actually do have a shot at hitting 700+ even though the timeframe is pretty short.
Note: I'm assuming that you did NOT get really lucky on those 15 on which you guessed. Did you get most of them wrong? If you got more than about 3 or 4 right, then I retract what I said. :)
The standard deviation from last MGMAT practice test to official test is about 50 points. (The standard deviation on the official test itself is about 30 points.)
Next, don't just sit down and do EVERYthing. Prioritize your study based on your strengths and weaknesses.
Read this:
http://www.beatthegmat.com/mba/2011/09/ ... e-mistakesNext, analyze your practice test using this:
http://www.manhattangmat.com/blog/index ... ice-tests/If you'd like, come back here and tell us the results of your analysis. (Note: do share an analysis with us, not just the raw data. Part of getting better is developing your ability to analyze your results - figure out what they mean and what you think you should do about them!)
Read these two timing articles and start doing what they say:
http://www.manhattangmat.com/blog/index ... to-win-it/http://www.manhattangmat.com/blog/index ... anagement/Finally, take a look at the How To Study section of this article:
http://www.manhattangmat.com/blog/index ... -the-gmat/