Each activity has 15 lessons Days A-O. Each lesson gets progressively harder. For my students on tier 2 are having difficulty recognizing teen numbers than I would start them on a later lesson within that activity. With my tier 3 students I will usually start them on Day A and continue through until Day O. If they are still having difficulty I will repeat the lessons that they have are having difficulty with. Also, you can use the lesson at a small group to introduce any of the concepts This can be done with all students. Let me know if this answers your question. If you have anymore questions, please don't hesitate to ask me. Thanks!