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by shashank.ism » Sat Feb 06, 2010 9:45 am
vscid the question is not clearly visible.please put it as an attachment or write it in forum.
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by ajith » Sat Feb 06, 2010 9:59 am
Probability that Baron finishes in top 3 = 3/8
Probability that HappyCynic finishes in top 3 = 7/17
Probability that Inamoata finishes in top 3 =1/3

Probability that all 3 will finish in top3 = 21/8*17*3
Probability that B,H finishes I does not = 3/8*7/17*2/3 = 42/8*17*3
Probability that B, I finishes H does not = 3/8*10/17*1/3 = 30/8*17*3
Probability that I,H finishes B does not = 5/8*7/17*1/3 = 35/8*17*3

Total probability = (21+42+30+35)/8*17*3 =128/8*17*3 = 16/51
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by shashank.ism » Sat Feb 06, 2010 10:05 am
ok I got the image downloaded..
so with reference to your problem...
for her not winning t-shirt . either case (i)two horses out 3 does not get in top or case(ii)all the 3 does not reach top 3.
we will exclude case (ii) as we have only 5 horses in race. so atleast 1 of three horses will surely reach in top 3.

so the required probability = (1-3/5)(1-7/10)(1/2) + (1-3/5)(7/10)(1-1/2) + (3/5)(1-7/10)(1-1/2)
= (2/5)(3/10)(1/2) + (2/5)(7/10)(1/2) + (3/5)(3/10)(1/2)
=6/100+14/100+9/100
= 29/100

so the answer is C.
as already explained why removed the case (ii). this is very interesting part in this question and many people would do this mistake if try to solve problem hastily.
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by ajith » Sat Feb 06, 2010 10:10 am
Only one problem; odds and probability are different

If an event has probability p the odds of it is p/(1-p)

and here we are given odds so we have to convert it to probability first

https://en.wikipedia.org/wiki/Odds

[might be a mistake from the question maker, he might have meant probability when he wrote odds]
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by vscid » Sat Feb 06, 2010 11:47 am
ajith wrote:Only one problem; odds and probability are different

If an event has probability p the odds of it is p/(1-p)

and here we are given odds so we have to convert it to probability first

https://en.wikipedia.org/wiki/Odds

[might be a mistake from the question maker, he might have meant probability when he wrote odds]
Its a very good point. Odds and probability are indeed different. However, I have come across many questions like this, which mean probability but write it as odds.
So is it always safe to assume that odds and probability are the same for GMAT purposes?
The GMAT is indeed adaptable. Whenever I answer RC, it proficiently 'adapts' itself to mark my 'right' answer 'wrong'.
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by Ian Stewart » Sat Feb 06, 2010 12:37 pm
vscid wrote:
ajith wrote:Only one problem; odds and probability are different

If an event has probability p the odds of it is p/(1-p)

and here we are given odds so we have to convert it to probability first

https://en.wikipedia.org/wiki/Odds

[might be a mistake from the question maker, he might have meant probability when he wrote odds]
Its a very good point. Odds and probability are indeed different. However, I have come across many questions like this, which mean probability but write it as odds.
So is it always safe to assume that odds and probability are the same for GMAT purposes?
As ajith points out, odds and probability are not the same thing. If the probability something will happen is 3/4, the odds in favour of it happening are 3 to 1, for example; odds are the ratio of the probability something happens to the probability it does not happen. Real GMAT questions always ask for probabilities, and not for odds, and you do not need to know what 'odds' are for the GMAT (though since they are used all the time in real life, it is a useful concept to understand).

Now, if you have some test prep book that uses the word 'odds' when it means to use 'probability', I'd be quite concerned about what other mistakes you might find in that book; that's quite a fundamental error, and the words 'probability' and 'odds' are certainly not interchangeable. Of course, if you continue to use that book, you'll need to try to guess what the book means when it uses these words.
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by vscid » Sat Feb 06, 2010 12:46 pm
Ian Stewart wrote:
vscid wrote:
ajith wrote:Only one problem; odds and probability are different

If an event has probability p the odds of it is p/(1-p)

and here we are given odds so we have to convert it to probability first

https://en.wikipedia.org/wiki/Odds

[might be a mistake from the question maker, he might have meant probability when he wrote odds]
Its a very good point. Odds and probability are indeed different. However, I have come across many questions like this, which mean probability but write it as odds.
So is it always safe to assume that odds and probability are the same for GMAT purposes?
As ajith points out, odds and probability are not the same thing. If the probability something will happen is 3/4, the odds in favour of it happening are 3 to 1, for example; odds are the ratio of the probability something happens to the probability it does not happen. Real GMAT questions always ask for probabilities, and not for odds, and you do not need to know what 'odds' are for the GMAT (though since they are used all the time in real life, it is a useful concept to understand).

Now, if you have some test prep book that uses the word 'odds' when it means to use 'probability', I'd be quite concerned about what other mistakes you might find in that book; that's quite a fundamental error, and the words 'probability' and 'odds' are certainly not interchangeable. Of course, if you continue to use that book, you'll need to try to guess what the book means when it uses these words.
Thanks for making this clear!
The GMAT is indeed adaptable. Whenever I answer RC, it proficiently 'adapts' itself to mark my 'right' answer 'wrong'.
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