Intro: Khan Academy is a great source of learning things and increasing your knowledge. It has got thousands of videos and explanations on different math, science, economics, physics, biology, history topics. Practice questions are included with instant feedback and step-by-step hints to sharpen your skills. The app supports offline content feature so you could save your favorite content and access that later without requiring internet connection.
Note: To download Khan Academy old versions APK scroll down the page or click here: Old Versions