# Spencer bought 3 books of stamps and mailed a package. It cost $4.50 to mail the package. How do you define a variable and write an expression to represent the total amount he spent at the post office? ##### 1 Answer Jun 23, 2016 #### Answer: Cost = $3x + $4.50 #### Explanation: We do not know the cost of each book of stamps, but whatever it was, Spencer bought 3 of them. Let's call the value we don't know, $x.

The money spent on the books of stamps is: 3 xx $x =$3x

He also spent money mailing the package, but that amount we do know.

His total cost is therefore the money spent on the books as well on the mailing.
Cost = $3x +$4.50

This is an expression which we cannot simplify.