A variable is an unknown number. It is represented by a letter, commonly  x. Variables can represent any quantity you don't know ... an amount, an age, a time, a price, or any other unknown value. A number sentence using a variable is called an expression. Here is an example of an expression with a variable in it: 2·x + 1 This means 'twice an unknown number, plus one'. Notice that we've used a dot for a times sign between the number 2 and the variable x. This is because an ordinary 'times' sign looks like an x, and we don't want to confuse them. In fact, you can leave out the dot if you want: 2x + 1  is the same as  2·x + 1 Both mean  'twice an unknown number, plus one' In the expression  2x + 1, the variable x is an unknown number. If we knew what it was, we could work out the expression. For example, suppose  x was 5. Then we could work out the expression like this: 2x + 1  =  2(5) + 1  =  11 Here are some more examples of expressions. See if you can say each expression in words. If you place your mouse cursor in the box to the right of each expression you will see the answer. 4x + 3 3x - 1 2y 6a + 5 Notice that we're using other letters besides x sometimes. You can use whatever letter you like for the unknown value, but the ones used most commonly include x, y, z, a, b, & c. Variables are usually written using lower case letters. (ie: x, not X) The unknown quantity can also represent something real. For example, if pencils are worth 10 cents each, you could make an expression for how much it would cost to buy some pencils. Let's make the variable  n  represent how many pencils you buy. The cost of the pencils you buy is  10n If you buy 3 pencils, it will cost you  10(3) = 30 cents. If you buy 5 pencils, it will cost you  10(5) = 50 cents. If you buy 20 pencils, it will cost you  10(20) = $2.00. Let's look at our first expression example one more time:        2x + 1 The variable is x. It represents an unknown number. The coefficients are the numbers you do know, in this case: 2 and 1 Now let's look at some more expressions and how to evaluate them Intro | Expressions | Evaluating | Simplifying | Equations | Quiz Resources