A variable is a symbol that represents a number. A variable can represent
any number. In the algebraic expression 27 + *b*, *b* is a variable.
Likewise, variables can appear in equations. In the algebraic
equation 11×*s* = 22, *s* is a variable. If we replace *s* with the number
2, the equation will be true, since 11×2 = 22.

We use variables in algebraic expressions when the quantity of something is
unknown. For example, if I want to talk about "Peter's monthly salary plus
$200," but I do not know his monthly salary, I might write *s* + 200, where *s*
represents Peter's monthly salary in dollars. Or, if I want to say, "My book
weighs three times as much as yours, plus 5 lbs. more," but I do not know how
much your book weighs, I might write that my book weighs 3*w* + 5, where *w*
represents the weight of your book in pounds.

We also use variables to represent unknowns in algebraic equations. Consider
the statement, "If Greg grows 2 inches, he will be 60 inches tall." This
statement says that Greg's current height plus 2 inches is equal to 60 inches.
We write *h* + 2 = 60, where *h* represents Greg's current height.

Algebraic expressions and algebraic equations can contain more than one
variable. For example, 2×(*h* + *w*) is an algebraic expression that contains two
variables, *h* and *w*.