This Problem Set is a part of the Lesson 14, Unit 3, Grade 5. In this activity, students look for number relationships before calculating, for example, to use the associative property or part-whole understanding. For examples, student solve 2/3 + 1/5 + 1/3 + 1 4/5 by associating the following mentally: (2/3 + 1/3) + (1/5 + 1 4/5).