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 $x.$ $$4x=8$$ In this case $x$ is a variable in an equation that can be solved to identify the number. Alternatively, a variable can be used to represent a quantity that changes. Here is another example. $$\begin{array}{c}\text{Enrique’s weekly income varies, depending}\\ \text{on the number of hours he works.}\end{array}$$ In this case, Enrique's weekly income could be different from one week to the next, based on how much he works. The use of a variable here is appropriate because it is possible that the number of hours worked each week changes.