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A low-pressure weather system is approaching Plainville; [#permalink]

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07 Mar 2006, 09:27

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A

B

C

D

E

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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

if: percentage of spring rainfalls in Plainville that are thundershowers = 90 %

p(thundershowers) = 0.70 * 0.90

if: percentage of spring rainfalls in Plainville that are thundershowers = 30% p(thundershowers) = 0.70 * 0.30

But can't I use the same arguement for C?

" thundershowers, which sometimes result from low-pressure systems"

What if the % of thundershowers from low pressure is .1 or .9?

Exactly my thoughts.
If percentage of thunderstorms from low pressure in Plainville is 70, then the probability of thunderstorm will be .7
B has 2 links to get the probability
C has direct link for the probability.
Still not sure if B is the correct choice.

B. I was torn b/w B and C. The reason I thought C was wrong was b/c the question asks, "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?" --- Meaning what's the probability there will be a thunderstorm in SPRING (soon). The answer choice that best helps us answer that question is the one which tells us the pct of spring rainfalls that lead to thunderstorms. We already know the pct of low pressure systems that lead to rainfall(70%).

Even though B is two steps away from the initial event (low pressure systems), it gives the best information. C could tell us the percentage of thunderstorms that occur from low pressure systems all year long, which is not as relevant as B. Just my 2 cents.

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