The ancient Persians, who invented "algebra," found that using "X" was a BIG mathematical help in solving problems:
PERSIAN WIFE (suspiciously): How much have you had to drink?
PERSIAN HUSBAND: I had "X" beers.
PERSIAN WIFE: Well, how much THAT?
PERSIAN HUSBAND: It's a (burp) variable.
PERSIAN WIFE (not wanting to look stupid): Well, okay, then.
Historical footnote: Several years later, when the ancient Romans invented Roman numbers, and it turned out that "X" was actually equal to 10, there was BIG TROUBLE in Persia.
From Classic Dave Barry