One of the best known of these resources is Khan Academy, which was started by then–hedge fund manager Salman Khan as a series of online doodles and YouTube video lectures intended to teach math to his young relatives. Their immense popularity led him to quit his job in 2009 and devote himself to creating online educational materials, freely available to all. By May 2013, Khan Academy included more than 4,100 videos, most no more than a few minutes long, on subjects ranging from arithmetic to calculus to physics to art history. These videos had been viewed more than 250 million times, and the
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