Answer:

Algebraic expressions and formulae, such as y = x², a² + b² = c² and v = u + at, contain letters which can stand for many different numbers. These letters are the variables. Variables are useful in mathematics because they allow instructions to be specified in a general way. If one were forced to use actual values, then the instructions would only apply in a more narrow set of situations.

For example:
Specify a mathematical definition for finding the number twice that of any number : double(y) = y + y