facing escalating costs.

Let's suppose you had a personal business providing vaccinations to children, and the office building you are in requires that you validate parking. The children you see in the morning yield a profit of $5 each because parking is free. In the afternoon, the profit is only $3 each because you now have to validate for parking in the afternoons. After 4pm, you actually lose $1 per patient because parking becomes more expensive after that. Assuming you can't avoid the parking fees, would you stay late after 4pm, even though you'd be losing money for every patient you saw? How late would you stay?