This site gives you the opportunity to get to know and take full advantage of the educational portal created by Mr. Saul Khan in the US. It is an electronic video library for almost all subjects taught in Elementary and Secondary schools around the world. The mathematical part was also localized to the Czech language and is available at https://cs.khanacademy.org.
On our page, you will find a breakdown of teaching lessons along with their corresponding interactive exercises for MATHEMATICS according to the individual grades of Elementary and Secondary schools (Elementary schools are also localized in Czech, Secondary schools are still only in English).
Learn how to get started with Khan Academy here (in English):