Good luck! And sorry to hear of the security issue the last time you attempted to take the test. That must have been so frustrating.
A 640 is certainly not a bad score at all. It's a great baseline. Many courses (such as ours -- Economist
GMAT Tutor) offer a score guarantee, so that may be something worth looking into. We offer a score improvement guarantee of 50 or 70 points, or your money back. In fact, we just conducted a survey of our past Students and were so pleased to learn that about a quarter of our Students reported that they improved 90 points or more!
If you have any questions about GMAT prep options, or about the GMAT in general, please let me know.
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Economist GMAT Tutor