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## CR 1000

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f2001290 GMAT Destroyer!
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CR 1000 Sun Jun 03, 2007 10:26 am
Elapsed Time: 00:00
• Lap #[LAPCOUNT] ([LAPTIME])
A low-pressure weather system is approaching Plainville; rainfall results from about 70 percent of such systems in the Plainville area. Moreover, the current season, spring, is the time of year in which thundershowers, which sometimes result from low-pressure systems, are most likely to occur in Plainville.
Knowing which one of the following, in addition to the information above, would be most useful for determining the probability that Plainville will have a thundershower soon?
(A) the percentage of thundershowers in Plainville that occur in the spring
(B) the percentage of spring rainfalls in Plainville that are thundershowers
(C) the percentage of thundershowers in Plainville that result from low-pressure systems
(D) whether low-pressure systems in other areas are associated with rainfall
(E) whether Plainville has more or fewer thundershowers each spring than do nearby towns

Thanked by: viks
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Maciek Really wants to Beat The GMAT!
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Sat Aug 28, 2010 2:04 am
Hi all!

I've found lost CR question.

IMO B

Quote:
The probability of something being the case given that something else is the
case is called a conditional probability. We express the conditional probability
of A on B by writing Pr(A\B).
Source: Critical reasoning user manual

Pr(rainfall\low-pressure weather system) = 70%

(A) it is irrelevant

(B) it is most useful for determining the requested probability

(C) it is out of scope because spring is the time of year in which thundershowers are most likely to occur in Plainville

(D) it is out of scope because of other areas

(E) it is out of scope

hope it helps!

If you don't agree with my reasoning, post here! Thanks!

Best,
Maciek

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scholardream Rising GMAT Star
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Wed May 02, 2012 10:52 am
Hi Maciek,
I don't understand why C is out of scope, as thundershowers, which sometimes result from low-pressure systems and we have premise that A low-pressure weather system is approaching Plainville, therefore if we know C, we could determine the probability that Plainville will have a thundershower.

spartacus1412 Really wants to Beat The GMAT!
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Thu May 03, 2012 6:45 am
I agree with scholardream.. I would Prefer C

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ice_rush GMAT Destroyer!
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Thu May 03, 2012 1:35 pm
agreed - the answer should be (B). tricky question.

heymayank08 Really wants to Beat The GMAT!
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Sun May 06, 2012 8:04 pm
spartacus1412 wrote:
I agree with scholardream.. I would Prefer C
+1 C pls explain why is it out of scope..

ice_rush GMAT Destroyer!
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Sun May 06, 2012 8:40 pm
We know that thundershowers are most likely to occur in spring. So whether thundershowers result from low-pressure systems or NOT does not help us in determining the likelihood of thundershowers. Also, choice (C) does not mention anything about thundershowers in spring. We need an answer that ties rainfall to thundershowers and only choice (B) does that.

hope this helps

confuse mind GMAT Destroyer!
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Wed May 09, 2012 9:53 pm
I will go for A since this is exactly what is asked in the question.
The information given in the stimulus is not used if I go with option A, and I feel it has been given to mislead.

B - the percentage of spring rainfalls in Plainville that are thundershowers
there can be a thundershower without rainfall as well. Secondly, there can be rainfall for reasons other than low-pressure and thus it cannot give correct probability or rather will give less probability than the correct

C - the percentage of thundershowers in Plainville that result from low-pressure systems
thundershowers can result from reasons other than low-pressure and not sufficient to calculate the probability

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