A variable is a symbol used to represent an unknown quantity. Often, variables represent fixed but unknown numbers. Variables are usually denoted with letters such as $x.$ In this case, $x$ is a variable in an equation. Solving this equation for $x$ will identify the value of $x$ that makes the equation true. Alternatively, a variable can be used to represent a quantity that changes. In this case, Izabella's weekly income could be different every week, depending on the number of hours she works. Therefore, the use of a variable is appropriate. Let $x$ be the hours that Izabella works in a week. Her income can be written by adding the fixed $\$100$ to the $\$4$ per hour.