Lesson 6

The purpose of this lesson is for students to add a two-digit number to a three-digit number that requires composing a ten when adding by place.

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 that make the most sense to them to add which may include counting or adding-on by place or adding tens and tens and ones and ones. They may also use whichever representations help them make sense of each sum, including number lines or base-ten blocks. The activity and lesson syntheses focus on using known sums of 10 to anticipate when a ten is composed when adding by place. This understanding will be used in upcoming lessons as students add two three-digit numbers by place and anticipate when units may need to be composed.

• Create a set of cards from the blackline master for each group of 3.

In upcoming lessons, students will add using methods based on place value that involve composing both hundreds and tens. How does today's focus on using what students know about combinations of numbers that make a ten help prepare them for this work?