Lesson objective: Apply strategies to solve addition problems with four two-digit addends.

This lesson provides an opportunity for students to apply their knowledge and understanding of addition with many addends to a real-life situation. Students are asked to find the unknown number of swimmers in Group B based on information they are given, such as total number of swimmers and swimmers in Groups A, C, and D.

Key Concept students will use:

- a group of addends always has the same sum no matter what strategy we use to add them.

Skills students will use:

- adding multiples of ten (Grade 1, Unit 11)
- an understanding of place value to add (Grade 1, Unit 14)
- applying properties of operations to solve problems (Grade 1, Unit 15)

Students engage in Mathematical Practice 1 (Make sense of problems and persevere in solving them) as they solve problems with a missing addend. Mathematical Practice 8 (Look for and express regularity in repeated reasoning) is also featured in this lesson as students use strategies they have practiced in addition problems with two addends and apply them to situations that involve three and four addends.

**Key vocabulary: **

- addend
- compose
- decompose
- strategy