Lesson objective: Solve addition and subtraction problems where both "parts" are unknown.

This lesson provides an opportunity for students to apply their knowledge and understanding of addition and subtraction to a real-life situation. Students are asked to figure out how many goldfish and clownfish are in a tank when there are 19 fish in all.

Key Concept students will use:

- We can use strategies such as making ten, counting on, decomposing a number leading to a ten, and the relationship between addition and subtraction to add and subtract within 20.

Skills students will use:

- Understand addition and subtraction are related
- Apply properties of operations as strategies to add and subtract (Grade 1, Unit 6)
- Understand subtraction as an unknown-addend problem (Grade 1, Unit 6)

Students engage in Mathematical Practice 4 (reason abstractly and quantitatively) as they solve problems with both parts unknown.

**Key vocabulary: **

**Special materials needed:**

- Part Part Total Worksheet (optional)
- counters or cubes (optional)