This Problem Set is a part of the Lesson 15, Unit 8, Grade 2. In this lesson, students apply their subtraction skills to solve problems involving time intervals. Given two times, they must calculate how much time has passed between them, whether in whole hours or a half hour (e.g., the elapsed time between 3:00 p.m. and 7:00 p.m. or 6:30 a.m. and 7:00 a.m.).