Algebra uses letters, called variables, to represent numbers that correspond to specific values. Algebraic variables can represent the unknown and what you’re solving for in an algebra problem, as well as known or set values.
Usually, if you see letters toward the beginning of the alphabet in a problem, such as a, b, or c, they represent known or set values, and the letters toward the end of the alphabet, such as x, y, or z, represent the unknowns, things that can change, or what you’re solving for.
Here are some of the more commonly used variables:

n often represents some unknown quantity or number — probably because n is the first letter in number.

x is often the variable you solve for, maybe because it’s a letter of mystery. The letter x also is used to indicate multiplication (×). You have to be clear when you use an x, that you don't mean multiply.

C and k are two of the more popular letters used for representing known amounts or constants. The letter C is used frequently in calculus and physics, and it’s capitalized in those cases — probably due to tradition.
The letters that represent variables and numbers are usually lowercase: a, b, c, and so on. Capitalized letters are used most commonly to represent the answer in a formula; for example, the capital A for area of a circle equals pi times the radius squared, A = πr2.