This Problem Set is a part of the Lesson 1, Unit 3, Grade 3. In this lesson, students study the commutativity of familiar facts that use units of 2, 3, 4, 5, and 10 to discover that they already know some facts with units of 6, 7, 8, and 9. For example, students recognize that if they know 3 × 6 = 18, then they also know 6 × 3 = 18. They write out familiar facts and those known through commutativity, organizing them in rows and columns to form the beginning of a multiplication table.