Massachusetts Department of Elementary and Secondary Education 45 min(s)

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.