Khan Academy

Khan Academy is a non-profit^[6] educational organization created in 2006 by educator Salman Khan with a goal of creating an accessible place for people to be educated.^[7] The organization produces short lectures in the form of YouTube videos.^[8] Its website also includes supplementary practice exercises and tools for educators. All resources are available to users of the website. The website and its content are provided mainly in English, but are also available in other languages like Bengali, Hindi and Spanish.

https://en.wikipedia.org/wiki/Khan_Academy

Would you choose to build a house on top of an unfinished foundation? Of course not. Why, then, do we rush students through education when they haven't always grasped the basics? Yes, it's complicated, but educator Sal Khan shares his plan to turn struggling students into scholars by helping them master concepts at their own pace.

With the backing of Gates and Google, Khan Academy and its free online educational videos are moving into the classroom and across the world. Their goal: to revolutionize how we teach and learn.

https://www.khanacademy.org/