The word ‘variable’ means something that can vary, i.e., change. The value of a variable is not fixed. We use a variable to represent a number and denote it by any letter such as l, m, n, p, x, y, z etc. A variable allows us to express relation in any practical situation and to express many common rules and properties of geometry, algebra etc. An expression with a variable, constants and the sign of equality (=) is called an equation. The value of the variable which satisfies the equation is called a solution of the equation.