Lesson objective: Understand that we can represent problems using equations with letters standing for the unknown quantity.

Students bring prior knowledge of writing equations from 3.OA.D.8. This prior knowledge is extended to replacing boxes, question marks, and symbols as students use letters to represent an unknown value. A conceptual challenge students may encounter is relying too heavily on rules and procedures.

The concept is developed through work with equations, in which students make meaning for the letters or unknowns and the mathematical symbols. Students will contextualize the problem by connecting the letters and symbols back to the problem situation.

This work helps students deepen their understanding of equivalence because they may represent the task multiple ways and, yet, all will result in the same outcome or answer for the unknown.

Students engage in Mathematical Practice 2 (reason abstractly and quantitatively) as they make connections between previous work of using boxes and symbols to represent an unknown quantity to using an abstract representation towards using a letter. In this lesson, students must make sense of the known and unknown quantities and how they relate to the problem.

**Key vocabulary:**

**Special materials needed:**