Probability!

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Ozlemg: Master | Next Rank: 500 Posts; Posts: 407; Joined: Tue Jan 25, 2011 9:19 amFollowed by: 7 members; Thanked: 25 times

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.

The more you suffer before the test, the less you will do so in the test!

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Source: Beat The GMAT — Problem Solving | Mon Jul 04, 2011 6:45 am

Frankenstein: Legendary Member; Posts: 1448; Joined: Tue May 17, 2011 9:55 amFollowed by: 53 members; Location: India; Thanked: 375 times

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

Cheers!

Things are not what they appear to be... nor are they otherwise

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Ozlemg: Master | Next Rank: 500 Posts; Posts: 407; Joined: Tue Jan 25, 2011 9:19 amFollowed by: 7 members; Thanked: 25 times

by Ozlemg » Mon Jul 04, 2011 7:33 am

Well done!

Could you offer me a good source to study probability?

Thank you indeed!

The more you suffer before the test, the less you will do so in the test!

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abhisays: Senior | Next Rank: 100 Posts; Posts: 54; Joined: Sun Nov 29, 2009 7:11 pm; Location: Bangalore; Thanked: 4 times

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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nafiul9090: Master | Next Rank: 500 Posts; Posts: 226; Joined: Thu Nov 25, 2010 12:19 amFollowed by: 2 members; Thanked: 3 times

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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