This Problem Set is a part of the Lesson 12, Unit 6, Grade 1. In this lesson, students add a pair of two-digit numbers when the ones digits have a sum less than or equal to 10 using strategies developed in previous units. For example, when adding 47 + 23, students may choose to decompose the second addend into 20 and 3, add 20 to 47, making 67, and then add the remaining ones. Or they may choose to add the ones to the first addend and then add on the remaining tens.