Answer: The marginal benefit of $2500 is greater than the marginal cost of $1700 for the new waitress.
In economics, a rational decision is a decision where the total benefit equals or exceeds the costs associated with that decision.
In this case, Carroll's diner pays the new waitress $1700, but is able to earn $2500 by serving customers more quickly. Since the diner earns $800 (2500 -1700) more than the cost paid to the waitress, we can say that hiring an additional waitress is a rational economic decision.