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Author Topic: Logical Reasoning- Help with "Venn Diagram" questions  (Read 2339 times)

Silversoma

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Logical Reasoning- Help with "Venn Diagram" questions
« on: August 01, 2004, 01:01:20 PM »
Okay, so I'm finding that I am really good at all the different types of Logical Reasoning questions, EXCEPT for those that I describe as "Venn Diagram" questions.  For illustration purposes, here is a sample question of what I am talking about:

Preptest #23, Section 2, question 10:

All bridges built from 1950 to 1960 are in serious need of rehabilitation.  Some bridges constructed in this period, however, were built according to faulty engineering design.  That is the bad news.  The good news is that at least some bridges in serious need of rehabilitation are not suspension bridges, since no suspension bridges are among the bridges that were built according to faulty engineering design.

If the statements above are true, then, on the basis of those statements, which one of the following must also be true?

A) Some suspension bridges are not in serious need of rehabilitation.
B) Some suspension bridges are in serious need of rehabilitation.
C) Some bridges that were built according to faulty engineering design are in serious need
   of rehabilitation.
D) Some bridges built from 1950 to 1960 are not in serious need of rehabilitation.
E) Some bridges that were built according to faulty engineering design are not bridges
   other than suspension bridges.

My natural instinct is to start drawing Venn diagrams, but this technique obviously doesn't work, as I have proven to myself over and over.  Yet, I cannot seem to come up with a better strategy for solving these problems!  This is a worrisome problem for me, especially after writing preptest #23, which seemed to have at least 3 of these questions in one LR section.  Ahhh!

Does anyone have a better method for approaching these types of LR questions?  I am really stumped here.

Thanks for all your help!
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fobby lawyer

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Re: Logical Reasoning- Help with "Venn Diagram" questions
« Reply #1 on: August 01, 2004, 03:03:57 PM »
do not draw Venn Diagrams for LSAT purposes.  They are time consuming and possibly misleading for some trickier questoins. 

Here is how I would diagram this question using simplified logic notations:

(1) B (1950/60 bridges) --> NR (Need Rehab)
(2) B SOME F (Faulty designed bridges)
(3) SB (Suspension Bridges)--> ~ F (not Faulty design)
(4) NR SOME ~SB (not SB)

We can combine (1) n (2):

(1) B -->  NR
(2) B SOME F, which is equivalent to F SOME B. 

NOTE: the terms of SOME statements are reversable; the terms of MOST statements can also be reversed BUT the quantifier MOST will change into SOME.
_____________________________ _________________
by linking up the conditional (1) to the term on the right of (F SOME B), we get F SOME NR, which is the answer choice (C)

NOTE: you can link up SOME or MOST statements with conditionals but only to the terms on the right of the SOME or MOST statements.


Matthew_24_24

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Re: Logical Reasoning- Help with "Venn Diagram" questions
« Reply #2 on: August 03, 2004, 10:03:38 PM »
Silversoma: try substituting numbers for the propositions...it often makes the answer jump out.


I use the number 10.  It makes it easy.

***"All bridges built from 1950 to 1960 are in serious need of rehabilitation."***

10 bridges, all need rehab.

***Some bridges constructed in this period, however, were built according to faulty engineering design.***

1-10 of the bridges have faulty enginnering.

***The good news is that at least some bridges in serious need of rehabilitation are not suspension bridges***

1-10 of the bridges are not suspension bridges

*** no suspension bridges are among the bridges that were built according to faulty engineering design***

0 suspension bridges are faulty engineering bridges.

Now we have this:

1) 10 bridges, all need rehab.
2) 1-10 of the bridges have faulty enginnering.
3) 1-10 of the bridges are not suspension bridges
4) suspension bridges are faulty engineering bridges.


Then i start working through the answer choices.

a) says that 1-10 bridges dont need rehab. WRONG. (point 1)
b) says 1-10 suspension bridges are in need or rehab (but do we need a suspension bridge? NO. (point 3)
c) says that the bridges that are of faulty engineering (1-10) are in need of rehab (all 10). This is your answer.

Thats how i do these questions. It works fast.

Matt