First, students will learn how to solve word problems by modeling each as an addition or subtraction problem. For addition problems, the sum will never be more than 20. For subtraction problems, the minuend will never be more than 20.
Next, students will learn how to solve word problems by modeling each as a missing addend problem, or a subtraction problem.
Next, give your students this challenge:
Up and Down by NRICH
Conclude by leading this investigation:
1.OA.A.1: Use addition and subtraction within 20 to solve word problems involving situations of adding to, taking from, putting together, taking apart, and comparing, with unknowns in all positions, e.g., by using objects, drawings, and equations with a symbol for the unknown number to represent the problem.