Up until the sixteenth century, math problems were written using only words. In the sixteenth century, the plus (+) and minus (-) made their first appearance. Rene Descartes ("I think, therefore I am"), was the first to use "X" as a variable all the time. He actually used letters at the end of the alphabet for unknown variables and letters at the beginning of the alphabet for known variables.

Why "X", one might ask? No one knows for sure. But there have been a few damn good guesses: It's simple to draw on the board, in the sand, etc. (the same reason why illiterate people use "X" to sign their name). In the Middle Ages, most educated aristocrats signed important papers with "X", because it was the sign of the apostle St. Andrew and implied a guarantee to live up to the promises in the document. I guess it just stuck. It was a sign of the times!