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Luis von Ahn holds a PhD in Computer Science from Carnegie Mellon University. His thesis, completed in 2005, was the first publication to use the term "human computation", referring to methods that combine human brainpower with computers to solve problems that neither could solve alone. In 2007, von Ahn invented reCAPTCHA,[26] a new form of CAPTCHA that also helps digitize books. In 2009, von Ahn and his graduate student Severin Hacker started developing Duolingo, which now has 46.6 million daily active users 46.6 million daily active users worldwide.