This Problem Set is a part of the Lesson 24, Unit 4, Grade 1. In this lesson, students interchangeably add sets of two-digit numbers, where the ones digits produce a sum less than or equal to 10. For example, when adding 17 + 13, students decompose the 13 into 10 and 3. Then they add 10 to 17, making 27, and add the remaining ones to find 27 + 3 = 30.