Here is a link to the video explanations to all of the quantitative/math problems in the Official Guide 11th Edition:http://www.gmatquantum.com/og11/
I have color coded the problems that are duplicates in Official Guide 12th
edition, and you can skip them if you have already finished the OG 12th
Edition. There are about 150 unique problems.
This may be helpful for students who need additional official GMAT practice questions, or students who have access to the 11th edition instead of the current version.
Free Video Explanations: OFFICIAL GUIDE FOR GMAT REVIEW 2015.