What does a variable represent in mathematics?

In mathematics, a variable represents a letter or symbol that stands for an unknown quantity or value. This flexibility allows variables to be used in equations and expressions to represent a range of possible values. For example, in the equation (x + 5 = 10), the variable (x) can take on the value of 5, which satisfies the equation. This ability to stand in for different values is essential in algebra and other areas of mathematics, as it enables the formulation of general rules and relationships.

The other options describe concepts that do not align with the definition of a variable. For instance, a constant is a fixed value that does not change, while an expression is a combination of numbers and operations rather than a symbol representing an unknown. An equation with no solutions presents a specific condition and does not relate to the representation of unknown quantities.