The apps, available for iOS and Android, are easy to use and contain over 100,000 adaptive practice questions that adjust in difficulty based on your results. In addition, if you struggle with a concept, it will recommend resources, like articles and videos, to help you. The resources are excellent. The dashboard shows you your progress.
This is a great way for students to study math and science concepts, whether for their own enrichment, extra help, or for studying.
CK-12 has over 100,000 teacher created digital textbooks available for free