tricky p. 2 maths question

This question has been asked a number of times. The text below is taken from my article on the same question. (http://hk.knowledge.yahoo.com/question/article?qid=6908122800145)

從會計學或經濟學角度去計算，損失都是同樣100元。其實問題只是王老闆收了100元假鈔，損失了多少。我將問題轉為以下兩個情況：(1)一天有個年輕人來到王老闆的店裏用100元假鈔換了100元零錢，再用換回來的21元買了一件成本18元的貨物，問王老闆在這次交易中損失了多少﹖(2)一天有個年輕人來到王老闆的店裏用100元假鈔換了100元零錢便走了，跟著另一個真顧客用21元買了一件成本18元的貨物，問王老闆在這次交易中損失了多少﹖情況(2)亳無疑問損失是100元，情況(1)與情況(2)的惟一分別是顧客不同，那麼情況(1)又怎可能損失比情況(2)少呢。有人會說情況(1)是兩項交易，換錢損失是100元，賣貨賺了3元，所以換錢單項損失是100元；而情況(2)是一項交易，換錢損失是100元，賣貨賺了3元，所以單項損失是97元。以上說法也不對，從會計學看情況(1)與情況(2)並無分別，被騙100元入呆帳，賣出貨物入銷售帳，無論賺了多少呆帳都是100元，這個是改不了的事實。從經濟學去看，答案中已經指出，貨物售價(不是成本)是要計算在損失內，賣給真正的顧客，王老闆可以收到21元，所以21元是王老闆的真正損失，損失總數也是100元(= 79 + 21)。

2012-11-17 00:54:53 補充：
王老闆當時沒有零錢,用那100元向街坊換了100元的零錢等等全是煙霧，與主題無關。

王老闆收了100元假鈔，失去79元的找續和21元的貨物，所以王老闆的損失是100元。

為甚麼貨物價值是21元而不是18元呢﹖因為如果賣給真正的顧客，王老闆可以收到21元，所以21元是王老闆的真正損失，這個也是經濟學機會成本的觀念。

2012-11-17 00:56:13 補充：
其實問題就係有人用100元假鈔換王老闆100元真鈔，你話王老闆損失幾多﹖

有3蚊利潤都係被騙100蚊。

2012-11-18 00:46:58 補充：
There is only one correct answer be it a P2 question or a graduate question. The school level has no bearing to the answer.

2012-11-18 00:51:00 補充：
Let me put it this way. If an item at cost $20 and selling for $30 is stolen from a store, how much is the loss to the store?

Insurance companies will pay only $20 and the store can purchase another and sell it for $30.

2012-11-18 00:56:20 補充：
It the store has no insurance, the store loses $20 for the good and $10 that could have been earned from selling the item.

2012-11-19 00:02:21 補充：
The questions you raised are true enough. That's exactly why social science disciplines, economics in particular, place "ceteris paribus" assumption (other things being equal) on their theories and hypothesis...

2012-11-19 00:04:01 補充：
... to rule out the effects of other factors that may sidetrack the relationship of the phenomenon being observed.

2012-11-19 00:07:59 補充：
To assume that other factors than the ones being observed have no bearing on the expected outcome is necessary for formulating useful predictions and explanations.

2012-11-19 00:09:59 補充：
That probably goes too far from the question of this thread. Anyway, limiting the factors to the given numbers is necessary for arriving at an answer.

從會計學或經濟學角度去計算，損失都是同樣100元?

This is another piece of language art of Garlic2010.

I'm wondering if this "proud owner 驕傲擁有人" do really understand what is opportunity cost and the basic accounting practices! Poor dumb!

2012-11-20 23:56:58 補充：
The total loss of this shop owner is $197.

Why?

$18 is the cost of goods. (According to IAS2)
$79 is the change to that customer. (It's a fact as cash goes down)
$100 is the loss of receiving a fake bill. (FACT- It's the owner's risk!)(It's unethical to pass the risk to an innocent.)

ATTENTION: THERE'S NOTHING ABOUT OPPORTUNITY COST!!!

The $3 is neither an actual gain nor an unrealized gain. In addition, the $3 isn't an opportunity cost because it isn't a cost to pursue that shop owner from being deceit. Put another way, there's no benefit from being deceit!

Focus on the money Mr Wong actually gave out. After the trade , the present is gone, and the money change is gone.

Money that Mr Wong lost are:
(1) the present's cost of $18
(2) the $79 he gave the young man as change.
Therefore, total loss in this trade = $18 + $79 = $97


2012-11-17 04:34:54 補充：
I think my simpler calculation on Mr Wong's loss is a suitable and correct one for the purpose of answering this arithmetic question.
I still have to thank garlic2010 for presenting an accounting/economics analysis on profit and loss. But it is not suitable for a P.2 question.

2012-11-17 04:41:13 補充：
~

2012-11-17 04:47:17 補充：
The above reference containing simple arguments in line with my answer is also suitable for P2 students to understand.

2012-11-17 04:54:48 補充：
參考原文:
"老闆係無左:79+18=97元 老闆既損失只係貸品買回來的價錢和找回年輕人的79元...................因:
老闆用假100元換左真100元,姐係有100蚊既收入,但換番假100元既時候只不過係比番換番來的真100元街坊,最後老闆係e件事上邊無損失~但多左張假100元
e加就可以解釋到老闆e加只係無左18元
另外,好多人會計埋佢賣21蚊搵番來果3元都係老闆既損失~其實係錯既~因為老闆買件貸品番來既時候係18元~21元只係老闆所定既價錢~e到都係錯覺之一= =
總結:假100元換真100元果件事同埋"21蚊"的事只係錯覺~好多人理解錯左~"

2012-11-18 18:04:06 補充：
If the vendor has insured against theft, deceit etc, we can say that he loses nothing.
If the vendor becomes emotionally disturbed and has to consult a psychologist, his loss will include the medical bill.

2012-11-18 18:05:49 補充：
If the vendor calls the police and spends two hours in the police station, his loss will include the two hours of time.
etc....