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 In this case 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. 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.