In algebra letters like: a, b, c...x, y, z are used to represent unknown numbers, and those letters are known as variable. If there is an unknown value in any sort of operations, once the real value of the variable is known, therefore it can be replaced by by the known value. Variables are frequently used in equations, formulas and mathematical rules For example:

x + 2 + 5; for x = 6

Let us replace x by its value:

6 + 2 + 5 = 13

Example 2

y + 4 + 2; for y = 3

Once again, let us replace y by its value.

3 + 4 + 2 = 9

Example 3

y = 4x + 3, for x = 2

Let's us replace x by its value

y = (4x2) + 3

y = 8 + 3

y = 11