This Problem Set is a part of the Lesson 16, Unit 4, Grade 1. In this lesson, students begin to focus on adding ones with ones or adding tens with tens. They recognize single-digit addition facts as they solve, for example, 15 + 2, 25 + 2, and 35 + 2. When adding 35 + 2, students see that they are adding 5 ones to 2 ones, while the tens remain unchanged, to make 3 tens 7 ones or 37.