Answer:
To solve this you would do the following:
We know her plan is $15/monthly, regardless of any additional texts/calls she makes.
If 1 minute = $.20 worth of money, we would simply multiply 30 (she made 30 minutes worth of calls) x $.20 ($.20 for each minute). The product is $6.
So, $15 + $6 = $21.
Unless the amount owed per text is mentioned and you forgot to include it in the title, you would not need to take this into consideration.
