Often people who are relatively comfortable doing math with numbers completely freak out when variables (you know, those letters that take the place of numbers) come into the picture. Things start looking much more abstract and complicated. You don't balance your checkbook with letters, do you? Nah. Numbers are just fine. Why do we need those letters? Wouldn't it be easier to just do without them? In some cases, the answer is yes, it would be easier to do without the letters. In many cases, though, the answer is no. Things can be much simpler when we deal with letters, as you'll see in this chapter. In addition to providing examples of when variables are useful, you'll learn how to develop general expressions for most of the procedures covered in the first four chapters.