We use alphabets like x, y, z etc., to denote variables. For example the length of a rectangle is denoted by 'l'. It takes different values in different rectangles. A formula is a relation between different variables formed using mathematical symbols. Any given condition can be translated into a formula using variables and mathematical symbols. Volume of a cuboid is the product of length, breadth and height. This is written as V = l x b x h. We say that this is the formula for the volume of the cuboid.
Formula
A formula is formed by using:
(a) mathematical symbols and variables (b) given conditions, and (c) simplification.
Change of subject of formula
In the formula 
I is called the subject of the formula.
To make 'P' the subject of the formula:
P x T x R = 100 x I
Summary
In a given sentence, the variables are replaced by letters and the different operations are replaced by their mathematical symbols. We now arrive at a formula.
