In previous grades, students developed fluency with sums of 10 and looked for ways to make a ten as a strategy for adding within 20. They used their understanding of these sums to make sense of composing a ten when adding by place, and as a strategy for adding within 100.

In this lesson, students use what they know about making a ten to find the sum of a three-digit number and a two-digit number. Throughout the lesson, students may use the methods of addition that make the most sense to them, which may include counting or adding on by place, or adding tens to tens and ones to ones. They may also use whichever representations help them make sense of each sum, including number lines or base-ten blocks. The Activity Synthesis and the Lesson Synthesis focus on using known sums of 10 to anticipate composing a ten when adding by place. This understanding will be used in upcoming lessons as students add 2 three-digit numbers by place and anticipate when units may need to be composed.