LSAT-18-2-18答案有问题

ztlbox

18. If there is a decrease in the number of homeless families, then either the number of available jobs has increased or else the cost of renting or purchasing housing has decreased. If the cost of renting or purchasing housing has decreased, then the supply of housing must be greater than the demand. Assume that there is a decrease in the number of homeless families.

According to the passage, which one of the following statements. CANNOT be true?

(A) The number of jobs has been decreasing.

(B) The cost of renting or purchasing housing has been decreasing, and housing has been decreasing, and housing supply exceeds demand.

(C) The number of jobs has been increasing, and the cost of renting or purchasing housing has been stable.

(D) The number of jobs has been decreasing, the cost of renting or purchasing housing has been decreasing, and housing demand exceeds supply.

(E) The number of jobs has been increasing, the cost of renting or purchasing housing has been increasing, and housing supply exceeds demand

答案是D,我选E.

lawyer_1

答案为D

1。decrease in the number of homeless families推出the number of available jobs increased / cost of renting or purchasing housing decreased（选言判断，否定一个必肯定另一个）。后者又推出the supply of housing greater than the demand。整个推理的起点是decrease in the number of homeless families. 所以the number of available jobs increased / cost of renting or purchasing housing decreased。D说前面是假，后面是真。至此是对的。但最后半句必错，因为cost of renting or purchasing housing decreased推出the supply of housing greater than the demand。而D却相反。故必错。

2。E可能对，也可能错。E说前半句队后半句错。至此是对的。最后半句可能对也可能错。因它是cost of renting or purchasing housing decreased推出the supply of housing greater than the demand的否命题。

ztlbox

谢谢斑竹的耐心回答，有两个地方还没搞懂

1，A or B must mean "A和非B" and "B和非A"吗？

2，条件是一升一降，如果变成都上升或都下降，该怎样判断呢

The number of jobs has been increasing, the cost of renting or purchasing housing has been increasing, and housing supply exceeds demand。

If the cost of renting or purchasing housing has decreased, then the supply of housing must be greater than the demand

第二句与文中最后一句话正好相反呀

请执教。

lawyer_1

1。对

2。相反，故为否命题。否命题不一定对，也不一定错。是POSSIBLE。条件是一升一降，如果变成都上升或都下降。都对，就像D，E的前两半句话，D都降，E都升，都不是CANNOT be true。E是错在最后半句话。其实你的问题在上面都可找到答案

ztlbox

以下是引用lawyer_1在2004-8-23 20:17:00的发言：

1。对

2。相反，故为否命题。否命题不一定对，也不一定错。是POSSIBLE。条件是一升一降，如果变成都上升或都下降。都对，就像D，E的前两半句话，D都降，E都升，都不是CANNOT be true。E是错在最后半句话。其实你的问题在上面都可找到答案

gg的逻辑功底很深厚嘛，要多向你学习。

想小声问一下否命题，A推B的否命题是非A推B，那么A推非B算不算呢。

多指教

lawyer_1

"A推B的否命题是非A推B，那么A推非B算不算"。非A推非B才是否命题。我的意思是原文If the cost of renting or purchasing housing has decreased, then the supply of housing must be greater than the demand.否定前半句（E所说，即cost of renting or purchasing housing has increased），不能否定后半句（因否命题不一定正确），即后半句then the supply of housing must be greater than the demand不能必错，即E的最后半句不能必错。

Bensontuo

ztlbox 发表于 2004-8-23 09:06
18. If there is a decrease in the number of homeless families, then either the number of available j ...

Can't not be true = Must be wrong.

So, we can think in a way that " If the argument of question is correct, it must be true that the answer must be wrong "

As a result, the necessary assumption of the argument is that the correct answer must be wrong.

So, what we are looking for is to negate the answer to see the fact that whether it could refute the argument. '

Core of the argument:

If the number of the homeless families decrease ( A ), either the number of available job has increased ( B ) or the cost of renting or buying the house has decreased ( C-1 or C -2 ).

If the cost of renting or buying the house has decreased ( C-1 or C-2 ), then the supply of housing must be greater than the demand. ( S > D )

A ---> ( B ) or ( C-1 or C-2 ) ---> ( S > D )

Answers

A. Based on our inference, b is not necessary to be happened only if ( C-1 or C-2 ) ---> ( S > D ) happened. Could be true

B. Just as the reasoning based on A, ( C-1 or C-2 ) ---> ( S>D ) is not necessary to be happened only if B happened. Could be true

C. Totally could be true.

D. If A, then ( C1 or C2 ), and if ( C1 or C2 ) ---> ( S > D ) Now, this one says ( D > S ). Mistaken Reverse. Must be false.

E. If A, then B and No C1 and No C2. ---> ( S > D )

One thing to remember is, when sufficient condition happens, Necessary condition will happen. However, we don't really need to have " one of the sufficient condition " to be happened to have our necessary condition happen.

Could be true.