Answer:
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.