87-90, pg. 285.
Sal works through 155 data sufficiency problems in the GMAC GMAT Review book (so using real GMAT problems). Buy the book, then watch the videos.
Khan Academy is dedicated to the goal of providing a free world-class education for everyone. Founded by Salman Amin Khan, the website features over 3,200 short-form videos that provide a basic introduction to a variety of topics that span from mathematics to current world events. The site is visited by over four million learners each month, and is widely considered an invaluable learning tool for individuals looking for a quick refresher on a subject. The videos are produced in a conversational style and specially designed to be easily viewed on a computer screen.
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