This Problem Set is a part of the Lesson 21, Unit 7, Grade 2. In this lesson, students represent whole numbers as lengths on a number line. They identify unknown numbers by using mental benchmarks or reference points (e.g., 5, 10, 25, 50) and intervals of 1, 5, or 10. Problems increase in complexity as students use their understanding of place value and the distance between positions to label points.