The free version of Mathway provides students with ready answers to math and chemistry problems at nearly any level. A subscription must be purchased to access step-by-step solutions that some students may need to understand the process of solving the problems, though.
Khan Academy’s app provides a comprehensive library of topics and skills that support Common Core goals. It includes a wealth of information particularly on middle school and high school classes, as well as useful topics for college students and adults.
CK-12 offers a comprehensive system of review resources and practice questions that will help students get ahead in school. The app does a thorough job of covering vital ideas in elementary math as well as middle and high school high school math and science.