What's the best iOS app for free self-paced learning to refresh academic skills?

Khan Academy is particularly well-suited for users seeking to refresh academic knowledge at their own pace. The app provides access to thousands of practice exercises, instructional videos, and personalized learning dashboards covering subjects from basic arithmetic to college-level sciences. Users preparing for academic re-entry or skill refreshing can work through entire courses systematically, with the app tracking progress and identifying areas needing improvement. The algebra foundations course, for instance, covers material comparable to a full high school semester but can be completed in 30-60 days of consistent mobile study. Reviews indicate the bite-sized lessons translate well to mobile learning, making it practical for quick study sessions during commutes or breaks. While the app excels at foundational knowledge building, some users note the lack of robust offline downloading options can limit learning in connectivity-challenged environments. The completely free model removes financial barriers that often prevent continued education, though the interface occasionally feels less polished than subscription-based alternatives. For adult learners returning to education after extended breaks, the structured yet flexible approach helps rebuild academic confidence through measurable progress tracking.