Why is x the most common variable?

x is the most common variable because x is usually used as a sign that signals that of a question. "x" is two diagonal lines, crossing out the wrong answer. which is a question, if someone doesn't know the right answer, they will most likely answer it wrong. So, gradually, "x" is "i don't know." so, that is marked or used as an unknown character.

^ this guys an idiot