If Maria designs a function to predict the amount of revenue, r, generated as a function of the number of people, n, who attend a fundraiser, explain why 13.5 would not be a member of this function's domain?

Only whole numbers of people, n, are assumed in the calculation.

Step-by-step explanation:

The function relating the number of people, n, to the revenue, r, implicity assumes that only whole people will purchase tickets.  13.5 would mean that 13 normal people and 1/2 of a person bought tickets.  This can't be in the domain, since the ticket saleperson would run away if 1/2 of a person showed up:  thus no ticket sale.

Actually, it may depend on how one defines a person.  Is it 1/2 physically, or 1/2 mentally?  I've personally seen the latter show up at some fundraisers.

2.83941151 would be the answer you're looking for! : )