probability-james : GMAT Problem Solving (PS)
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# probability-james

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01 Mar 2010, 19:15
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The probability that a convenience store has cans of iced tea in stock is 50%. If James stops by 3 convenience stores on his way to work, what is the probability that he will be able to buy a can of iced tea?
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01 Mar 2010, 19:22
1/2(finding in 1st store) + 1/4(not in first but in second) + 1/8(not in first and second but in 3rd) = 7/8
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02 Mar 2010, 04:36
chix475ntu wrote:
1/2(finding in 1st store) + 1/4(not in first but in second) + 1/8(not in first and second but in 3rd) = 7/8

chix475ntu,
Why is that we add probabilities here, whereas, if we want to find probability of not finding tea in any of the stores, we multiply them. (1/2 x 1/2 x 1/2) ?
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02 Mar 2010, 08:54
vscid wrote:
chix475ntu wrote:
1/2(finding in 1st store) + 1/4(not in first but in second) + 1/8(not in first and second but in 3rd) = 7/8

chix475ntu,
Why is that we add probabilities here, whereas, if we want to find probability of not finding tea in any of the stores, we multiply them. (1/2 x 1/2 x 1/2) ?

CASE II: the same problem can be solved in the following way = 1 - no tea in any of the 3 stores = 1 - 1/2 * 1/2 * 1/2 = 7/8.

CASE I: the probabilities are independent in the case I as I find the tea in the first store, then i am good. If not, then for hte second case, i dont find the tea in the first store and hten i find it in the second store etc.
CASE II: the 3 possibilities are dependent - as we say that I dont find the tea in 1st store, then 2nd store and then 3rd store.
** Also, this is case by case basis, just because i add the probabilities in this case doesnt mean that its always the case, I just did that as it applies to this case as I have 3 possibilities of finding tea(1st store, 2nd store, 3rd store) and by calculating one by one, the probabilities are different. Let me know if you want any further explanation.
Re: probability-james   [#permalink] 02 Mar 2010, 08:54
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