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Probability!

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by Ozlemg » Mon Jul 04, 2011 6:45 am
A certain bookstore sells only 10 different titles of books. A customer buys 3 different titles out of 10 titles of books offered by the store. Another customer walks into the store and randomly picks 2 different titles of books. What is the probability that the second customer picks the titles that the first one didnt?

A.7/35
B.7/15
C.15/35
D.28/60
E.14/15

OA will come later! I need a detailed explanation.
Last edited by Ozlemg on Mon Jul 04, 2011 7:06 am, edited 1 time in total.
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by Frankenstein » Mon Jul 04, 2011 6:51 am
Hi,
If the first guy has picked 3 titles, for the favorable event the second one has to chose 2 from the remaining 7 in 7C2 ways
Total number of cases for second person to pick is 10C2
so, probability is 7C2/10C2 = 7.6/10.9 = 7/15

Hence, B
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by Ozlemg » Mon Jul 04, 2011 7:33 am
Well done! :)
Could you offer me a good source to study probability?

Thank you indeed!
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by abhisays » Mon Jul 04, 2011 7:57 am
In most of the probability questions, we should try to find the no. of favorable cases and no. of total cases.

For example in this case.

no. of favorable cases = 7C2
and total no. of cases = 10C2

Hence the probability would be 7C2/10C2 i.e 7/15

So always remember

probability = favorable no. of event / total no. of events.

and to crack the probability questions, the concepts of permutations and combinations are very essential.
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by nafiul9090 » Tue Jul 05, 2011 10:59 pm
Frankenstein wrote:Hi,
If the first guy has picked 3 titles, for the favorable event the second one has to chose 2 from the remaining 7 in 7C2 ways
Total number of cases for second person to pick is 10C2
so, probability is 7C2/10C2 = 7.6/10.9 = 7/15

Hence, B
hello frank

i also ask the same question as the Ozlmg does....could you please suggest study materials on probality ??

regards nafi
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