It sounds like you need to spend more time internalizing the explanations. What I did while studying was I'd look at the answer, but not the explanation. Then, I would sit there as long as it took me to figure out how to get to that answer. Sometimes it might take me 20 minutes, but I'd always do it eventually. And, you can bet I never forgot how to do that problem!

Eventually, I knew how to do pretty much every math problem, simply because I had done one just like it before and had memorized an efficient way to solve it. The key to studying is to focus on quality (that is, taking your time and actually studying and analyzing every problem) rather than quantity (flying through as many practice problems and practice tests as possible).

Hope this helps - 640 to 700 isn't unreasonable, but it certainly takes quite a bit of effort.

THANKS for that. one question, im to take GMAT on march 18. currently im studying 3 hours per day. am i doing enough?

If anything, that's too much. That's over five months from now - a good guideline for most students is about two hours per day for two or three months, depending on your background and general aptitude for the GMAT.

With that long, you run the risk of burning out - just be careful.

Also, with that long, you might want to consider not even looking at GMAT problems unti January. In the meantime, you can set an awesome foundation in quant and verbal. For quant, go out and get an algebra textbook that's not GMAT specific. For verbal, read as much as you can. It's practically impossible to break 700 with a great foundation in basic math and English - doing well after adding the complexity of the GMAT requires solid fundamentals.