Standard 2.OA.B.2 Fluently Adding and Subtracting Within 20. Building Automaticity through Effective Practice. Activities in this packet will provide support for students to be fluent when adding and subtracting within 20 based on mental math strategies. Strategies are essential for conceptual understanding and procedural fluency. The activities included here are to provide effective practice along with recording student growth through self– or teacher monitoring. These activities are great for centers, guided math, RtI, extended learning, whole class, or individual practice.