Lesson 19

The purpose of this lesson is for students to analyze addition methods for adding within 20, then use those methods flexibly to find sums based on the numbers in a given expression.

In previous lessons, students decomposed an addend in order to make a ten and thought about sums as equivalent \(10 + n\) expressions. They applied the commutative and associative properties to find the sum more easily. Students looked for and used patterns in addition expressions (such as \(4 + 8 = 5 + 7\)). In this lesson, students continue adding within 20.

Students may use any method they choose, but are encouraged to think about methods which may work better for certain sums. In the first activity, students analyze three different methods for finding a sum, all of which include decomposing an addend to make a known sum. In the second activity, students find sums within 20 and share their thinking with a partner.

• Each group needs a set of the Compare Stage 2 Addition Cards cards from the previous lesson.

What methods are students using to find sums? Which methods were you expecting and which surprised you?