你刚刚买了一件新的Ford Taurus,花了20000美元，但是如果你自己卖掉Ford Taurus，最多能够得到15000美元。现在你了解到Toyota提供Camry,这个东西正常情况下卖25000美元，现价为20000美元。如果你在买Taurus前，得知你可以用20000美元买一个Camry，你将一定选择Camry。下列说法对吗？即从你的消费偏好来讲，你是一个理性的效用最大化的人，你一定不会卖掉Ford Taurus，之后去买Camry，请解释为什么？
卖掉后得到15，000美元，那么买camry需要20，000美元，需要再贴5，000美元。但是你买了一个25，000美元的东西，比20，000美元节省5，000美元。
这样做的结果是什么也没有增加。

谢谢LZ。今天我也问了下老师，老师给出如下解释，我感觉还是挺有用的。实在是被机会成本搞的很晕乎啊。还是非常谢谢LZ。The problem is not very clear about what the preferences are. I guess you
had a look at the solutions since they are online, but I do not find them
very explanatory.


-------------------(you can skip this part if you want...I started typing,
and realized this was not really useful for solving, but it can still be
some introduction) ----------------------------------------------

When you buy the Taurus, you incur an opportunity cost: since you pay 20k
$ for it, but then you can only get 15k $ from it if you sold it back, the
oppportunity cost is 5k $, i.e. what you lose by buying the car.

This might be a bit tricky to conceive but think of it this way: when you
buy something, you are not really losing any wealth, but only transferring
it (assuming you behave in a rational manner).

Take a famous painting for instance. Let's say you buy a Picasso for one
million euros. We assume that the value will remain stable for ever. And
also, you are completely indifferent to art, i.e. you do not get any
personal benefits from having a Picasso.
Then. you lose 1 million in money (which you give away), but you get 1
million in Picasso painting. You are as rich as before, the only
difference is that 1 million of your wealth is now in painting instead of
money.

(If you get it so far, we can then discuss liquidity issues if you are
interested...but this is really extra stuff, not needed at this point.)

Back to this car example: you give an amount of money worth 20k $, for a
car which will then only be worth 15k $, but you also extract personal
benefits from the car, which you can supposedly convert in monetary terms.
So, having the Taurus is worth : (15k $ + personal benefits)

Since you bought the Taurus and since you are rational, your (marginal)
benefit from having the Taurus must be greater than 5k $, so that the sum
be greater than 20k $.

----------------------------------------------------------------
---------------------------------------------------------------

Remember this: if you buy a car and you are rational, it means that the
benefits you get from the car are worth more than the price paid.

Also, remember that when you buy the Taurus, the Camry is also available
for 25 k$, but you still buy the Taurus.
So, this means the following:

(total benefits from Taurus) - 20k$ > (total benefits from Camry) - 25k$

<=> (TB Camry)-(TB Taurus) < 5k$


Now let us say you have the Taurus, you sell it back for 15k$, and buy the
Taurus at the special offer, for a price of 20k$.
Then you have to pay an extra 5k$.
The opportunity costs are the benefits you were getting from the Taurus,
and the extra 5k$.
The marginal benefits are the benefits you are going to get from the
Camry, and the money from the sale of the Taurus.

Your costs are then:
- price of the car (20k$).
- loss of benefits from the Taurus.

Your benefits are:
- money from the sale of the Taurus.
- benefits from the Camry.

Take the difference and check if it is positive...
Since we already showed that (TB Camry)-(TB Taurus) < 5k$, you would
actually lose from selling the Taurus and buying the Camry.

So, if you are rational, you should keep the Taurus.

Now why do you buy the Camry AND the Taurus?

You already bought the Taurus, which means that it was beneficial for you
the buy it. Your net satisfaction is positive.

Now, since you would have bought the Camry if both cars were available at
the same price, it means you would get an even higher satisfaction from
buying the Camry at 20k$, than from the Taurus.
So, you should also buy the Camry, and your net satisfaction will be even
higher.

2# nlm0402

囧，把版主说成LZ了~~~~