Final answer:
The relationship between the number of mix-ins and the cost of ice cream is a function because each number of mix-ins corresponds to one specific cost, satisfying the definition of a function in mathematics.
Step-by-step explanation:
The relationship between the number of mix-ins you choose and the cost of your ice cream at the shop can be considered a function. A function in mathematics is defined as a relation where each input (in this case, the number of mix-ins) is associated with exactly one output (the cost of your ice cream). Given that the mix-ins all cost the same amount and this is added to the fixed cost of the ice cream scoop, for each number of mix-ins there will be one specific cost. Hence, this satisfies the criterion for a function.
When we describe economic models using algebraic models, we often refer to them as functions because they illustrate how one thing changes in relation to another. For instance, similar to how Ethan's income in the question given can be expressed by a linear function (y = 25 + 20x), the cost of your ice cream can be similarly modeled when considering mix-ins and base price.