A package of paper towels contains 3 rolls. Each package of paper towels costs $2.79.

Question

A function of f(x) is written to represent the cost of purchasing.
What is the practical domain for the function?

All whole numbers
All positive
All whole numbers that are multiples of 3
All real numbers

Answer 1

well, towels usually come in whole rolls, right?
But the domain is for packages of towels, not individual rolls, so not multiples of 3.

Pick #1
because whole numbers are in fact always positive.