In this lesson, students learn that subtraction can be seen as an unknown addend problem and can be solved by using addition (related facts). Tools for modeling problem situations: Unifix cubes, two sided counters, tiles, number line. By the end of this lesson students will know and be able to create a situation equation to model a problem, and use related facts to create a solution equation. Use addition and subtraction within 20 to solve word problems involving situations of 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.