Which mathematical concept uses letters to represent the variables in an equation?

Algebra is the branch of mathematics that focuses on the use of symbols, typically letters, to represent variables in equations and expressions. In algebra, letters are often used to stand for numbers and can be manipulated just like regular numbers through various operations such as addition, subtraction, multiplication, and division. This abstraction allows for the formulation of general rules and principles that can be applied to solve a wide range of mathematical problems.

For instance, in the equation (x + 5 = 10), the letter (x) represents an unknown value. Algebra enables the solving of such equations to find the value of the variable, showcasing its purpose in expressing relationships between quantities. This foundational aspect of algebra is essential for understanding higher-level mathematics, as well as practical applications in engineering and sciences.

The other choices, while integral to mathematics, do not primarily focus on the representation of variables using letters. Geometry deals with the properties and relationships of shapes and spatial figures, calculus involves the study of change and motion through derivatives and integrals, and statistics primarily concerns the collection, analysis, interpretation, and presentation of data.