Some of my students miss classes for extended periods of time or prefer to utilize a computer tutorial that provides them with specific immediate feedback. If you look closely at the links in CK-12 flex books, you will find a tool called Braingenie. It is an open source tool that allows you to set customized goals that align to Ck-12 flex books within a video tutorial system. I offer this to students as an independent alternative to class work. Oddly enough, most of my students choose to work on assignments that we do together in class. It is nice to have this as a back up. I have found that some students respond well to this tool for difficult subjects. I also like to assign how it tracks students that are at different places at the same time.