This Problem Set is a part of the Lesson 19, Unit 1, Grade 1. This lesson introduces the commutative property. Students write two expressions to represent the same quantity and place them in the context of an equation. For example, when examining a group of 4 red counters and 3 white counters, they write the expressions 4 + 3 and 4 + 3 to represent the total number of counters, and then create the equation 4 + 3 = 3 + 4.