This Fluency Activity is a part of the Lesson 2, Unit 1, Grade 4. Practicing skip-counting on the number line builds a foundation for accessing higher-order concepts throughout the year. Direct students to count by threes forward and backward to 36, focusing on the crossing-ten transitions. The purpose of focusing on crossing the ten transitions is to help students make the connection that, for example, when adding 3 to 9, 9 + 1 is 10, and then 2 more is 12. There is a similar purpose in counting down by threes.