Answer:
The total cost of mailing a letter is expressed by:

The cost for mailing a four-ounce letter is: $1.10
Explanation:
First, you have a fixed cost, independent of any other variable, which is
.
Then you have a cost that is dependent on the weight, expressed in ounces, which is
per ounce. This means that if you had
ounces, then your cost would be duplicated, becoming
, and so on and so forth. Let's define the weight as
.
The total cost of mailing a letter equals the base charge (or the fixed cost) plus the cost of the number of ounces that our letter weighs.
We can express this statement in mathematical terms:

Now, we want to know how much it would cost us to mail a letter that weighs
ounces. That means we know the value of the
in our expression, so we can substitute it and solve the expression:



It would cost us $1.10 to send a 4 ounce letter.