In previous lessons, students practiced composing and decomposing three-digit numbers in different ways. This lesson connects composing and decomposing numbers to methods for adding and subtracting numbers within 1,000. The first activity encourages students to consider which sums and differences within 1,000 would be least and most challenging to find. Students are encouraged to think about what might make finding the values of some expressions more challenging. They share the methods they use to find these values. The second activity focuses only on sums within 100 and encourages students to use and share methods that help them fluently find sums and differences.

How did students reason about which sums and differences were least and most challenging to find? What does this tell you about students’ understanding of place value and numbers?