- Topics At a Glance
**Variables**- Variables as Unknown Quantities
- Variable Notations
- Constants
- Expressions and Equations
- Rearranging Expressions
- Commutative Properties
- Associative Properties
- Distributive Properties
- Factoring (Distributive Property in Reverse)
- Combining Like Terms
- Eliminating Parentheses
- Simplifying
- Equations, Functions, and Formulas
- Equations
- Functions
- Independent and Dependent Variables
- Formulas
- Applications to Toolbox
- Evaluating Expressions by Substitution
- Evaluating Formulas by Substitution
- Geometric Formulas
- Four-Sided Shapes
- Three-Sided Shapes
- Circles
- Unit Conversion
- Temperatures
- Weights
- Distances and Speeds
- Money
- In the Real World
- I Like Abstract Stuff; Why Should I Care?
- How to Solve a Math Problem

A **variable** is a letter, such as *x* or *y*, that represents a number. Another way of thinking about it is that a variable is "code" for a number. Mathematicians can spend hours at a party talking to one another in variables and no one will have the slightest clue what they are talking about.

The most common variable used is *x*. When we have multiple variables, we may use *x*, *y* and *z*. However, any letter will work, as long as we are consistent with it. We tend to use *x*, *y* and *z* because we feel bad for them having to hang out at the back of the alphabet all the time. This is their time to shine.