This Problem Set is a part of the Lesson 27, Unit 2, Grade 1. In this lesson, students solve "result-unknown" problems. They move through a progression from problems with teens decomposed or composed using 1 ten and some ones (such as 13 - 3 or 16 - 4) to problems where they find the hidden ten (such as 8 + 6 or 12 - 5).