Just took my GMAT and scored 680 (Quant 48, Verbal 34). I messed up my first exam and the worst part was that I knew it after I finished quant. I think In all the excitement I forgot to remember that I had to be careful during first 15 question and double check every answer. The result was that I never got to hard questions. All the questions I faced were so easy that I finished quant in 50 minutes. So it never tested my real ability. So Yes it is very important that you get the first 15 questions right. No matter how good you are, you cannot afford to make mistake in first 15 question. In my next test yesterday, this is the only thing that worked for me. So even after I ran out of time and just had to guess last three questions I got decent score.
For me it was a real difficult to start with as I finished my highschool in 1986. More than twenty years since I did any real math. Worst part is I am a native Indian and in India highschool means standard 10, After that you have to chose between biological sciences or mathematics. So I never studied probality or permutations and combinations that are part of class 11-12 mathmatics and that are hard questions on GMAT. Here these forums have helped me a lot specially beat the GMAt and Manhattan GMAT. These questions were real hard for me. Beside this, anybody with a long gap in math practice and who is going to take the GMAT, Here are two very useful math websites that I used to get aquainted with the general principals of math that are tested on GMAT and offer lot of practice questions on each topic ( I don't know how useful these are for other people)
https://www.algebralab.org/lessons/lesson.aspx
https://www.purplemath.com/modules/index.htm.
My whole preparation for GMAT starting from practice with these website to actual test took 3 months. After I finished with every concept in these sites (which took a month, since I am working full time) I practiced from OG11, princeton review and kaplan.
In addition I practised every test posted on forums. Took both practice tests in PowerPrep each atleast four times. This way you get to see all different set of questions. Permutation combination questions posted on beat the GMAT forum and Manhattan GMAT helped me a lot.
OG 11 last 100 questions I did three time, kept a record and marked on the questions on which I made a mistake everytime. that indicated that I had problem in reasoning that questions, while others usually meant carelessness.
Verbal I did not study at all. I am Ph.D. in immunology and have to do lot of reading everyday. However, being a non-native English speaker is a disadvantage, but I figured since I read so much everday, I am not going to improve any further than where I am now. My strongest ability is critical reasoning (almost never get any question wrong) and my weakest part is RC. But the the truth is everytime in simulated test I scored between 67-69 percentile on verbal. In both my GMAT, test I scored exactly 69 percentile. I have lot of study and practice material (more than 250 questions) downloaded from different websites. Anybody living in NJ who wants them I will happy to give out. After looking at my Aug score, I am just happy to get this score.
I just want to thank everybody on this forum that helped me a lot in my preparation.