In this lesson, students find the value of sums and unknown addends from memory in expressions with values within 20. In the first activity, students complete an inventory of sums within 20 and identify those sums that they do not know yet from memory. In the second activity, they continue to practice fluency with addition and subtraction within 20.

Throughout the lesson, encourage students to share the methods they use to recall or find a sum or difference fluently. These include methods based on using the relationship between addition and subtraction, decomposing to get to a ten, and creating expressions that are equivalent, but easier to find mentally.

Which sums within 20 did students show they are still working on? How can you make time for practice with these sums during the school day?