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Whether you're a student, teacher, homeschooler, principal, adult returning to the classroom after 20 years, or a friendly alien trying to get a leg up in earthly biology — Khan Academy's personalized learning library is available to you, for free.

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- Pick up where you left off: Tailored to your current learning level, our mastery system gives instant feedback and recommendations on exactly which skills and videos to try next. And, if you choose to create a free account, your learning syncs with http://khanacademy.org, so your progress is always up-to-date across all your devices.

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