Lesson objective: Understand the strategies of counting on and counting back to find a difference within 20.

Students bring prior knowledge of subtracting within ten from K.OA.A.2, Solve addition and subtraction word problems, and add and subtract within 10, e.g., by using objects or drawings to represent the problem . This prior knowledge is extended to numbers within 20 as students use contexts and models to subtract numbers within 20. A conceptual challenge students may encounter is counting backwards and keeping track of the count.

The concept is developed through work with a number path, which shows a linear model of numbers that allow students to to count up and back at the same time they are building a visual to keep track of the counts.

This work helps students deepen their understanding of operations because they are working with a meaningful context for subtraction and learning strategies for subtraction.

Students engage in Mathematical Practice 2, Reason abstractly and quantitativley as they take numbers from a real context and use a number path and number sentences to represent the mathematics in the problem.

**Key vocabulary:**

- count back
- count up
- difference
- number path
- strategies
- subtract

**Special materials needed:**

- For solving the task, have available,
- number path 0 - 20
- counters