3. Khan Academy

This amazing organization has more than 3000 video lessons covering everything from basic arithmetic and algebra to differential equations, cosmology, physics, chemistry, biology and even art history. Their mission is to change education for the better by providing world-class education to anyone, anywhere and they also do all that for free. You can use their extensive library and find all the information you need on pretty much everything. The best thing about this place is the fact that you are able to leave comments or ask questions if you don’t understand something. Discover more at khanacademy.org.

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