Lesson objective: Understand that a variable represents an unknown quantity in an equation.

Students bring prior knowledge of representing an unknown quantity with a symbol or picture in an equation from 1.OA.A.2. This prior knowledge is extended to using a variable to represent the unknown as students solve two-step word problems. A conceptual challenge students may encounter is understanding where to put the variable in the equation.

The concept is developed through work with equations, which helps students understand where to put the variable to represent an unknown quantity.

This work helps students deepen their understanding of equivalence because students will use their knowledge of the meaning of the equal sign when deciding where to put the variable in an equation.

Students engage in Mathematical Practice 2 (Reason abstractly and quantitatively) as they create logical representations of the problem at hand, including a variable for the unknown quantity.

**Key vocabulary:**

- equation
- unknown quantity
- variable