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?
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,
The money spent on the books of stamps is:
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.
This is an expression which we cannot simplify.