曼昆经济学原理中的一条：理性人考虑边际量.求各位大神给一个可以用这条原理分析的案例,最好附上分析.

英文版的案例,不明白的话直接用google翻译把英文翻译成中文.
PRINCIPLE #3:RATIONAL PEOPLE THINK AT THE MARGIN
Decisions in life are rarely black and white but usually involve shades of gray.
When it’s time for dinner,the decision you face is not between fasting or eating
like a pig,but whether to take that extra spoonful of mashed potatoes.When exams
roll around,your decision is not between blowing them off or studying 24
hours a day,but whether to spend an extra hour reviewing your notes instead of
watching TV.Economists use the term marginal changes to describe small incremental
adjustments to an existing plan of action.Keep in mind that “margin”
means “edge,” so marginal changes are adjustments around the edges of what you
are doing.
In many situations,people make the best decisions by thinking at the margin.
Suppose,for instance,that you asked a friend for advice about how many years to
stay in school.If he were to compare for you the lifestyle of a person with a Ph.D.
to that of a grade school dropout,you might complain that this comparison is not
helpful for your decision.You have some education already and most likely are
deciding whether to spend an extra year or two in school.To make this decision,
you need to know the additional benefits that an extra year in school would offer
(higher wages throughout life and the sheer joy of learning) and the additional
costs that you would incur (tuition and the forgone wages while you’re in school).
By comparing these marginal benefits and marginal costs,you can evaluate whether
the extra year is worthwhile.
As another example,consider an airline deciding how much to charge passengers
who fly standby.Suppose that flying a 200-seat plane across the country costs
the airline $100,000.In this case,the average cost of each seat is $100,000/200,
which is $500.One might be tempted to conclude that the airline should never
sell a ticket for less than $500.In fact,however,the airline can raise its profits by
thinking at the margin.Imagine that a plane is about to take off with ten empty
seats,and a standby passenger is waiting at the gate willing to pay $300 for a seat.
Should the airline sell it to him?Of course it should.If the plane has empty seats,
the cost of adding one more passenger is minuscule.Although the average cost of
flying a passenger is $500,the marginal cost is merely the cost of the bag of peanuts
and can of soda that the extra passenger will consume.As long as the standby passenger
pays more than the marginal cost,selling him a ticket is profitable.
As these examples show,individuals and firms can make better decisions by
thinking at the margin.A rational decisionmaker takes an action if and only if the
marginal benefit of the action exceeds the marginal cost.