Not sure if people know about this, but there are YouTube videos of all the Quantitative questions with answer explanation on YouTube. Just do a search "gmat : math 1". You will find the series of videos (Over 100 that covers the Quantitative including Data Sufficiency). Someone actually shows you how to do the problem. I happened to run into this on YouTube and it's has helped me tremendously. I had a very short time frame to study (2 weeks) and took my GMAT earlier this week. I got a 620, enough to get me into the MBA school I had targetted.
Albeit that it's my 2nd time taking the GMAT, I felt like these videos explained step by step approach to how to solve each problem. I learn better visually and a lot of times the explanation in the Official Guide confuses me. Just wanted to share this information and hope it will help others. Below is a link to the first video, but other "relevant" will show up and just follow it chronologically. http://www.youtube.com/view_play_list?p ... %3A+math+1