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04 Nov 2009, 18:31
Kudos
Bookmarks
Hi there,
I'm working on my last weak area before I am going to take the GMAT: PROBABILITY AND COMBINATORICS. So I apologize in advance if some of the questions I'm going to ask you guys are too easy.
QST: In a box with 10 blocks, 3 of which are red, what is the probability of picking a red block on each of your first two tries? Assume that you do NOT replace the first block after you have picked it.
ANS: I'm aware of the traditional way to solve problems like this (probability trees). By doing so, the answer is 3/10 * 2/9 = 1/15. Pretty straight forward.
Now, I want to also be able to solve this kind of problem with a counting method (combinatorics). So I thought I can solve it like this:
(3*2)/(10C2)
Unfortunately, I get the wrong result (2/15). This is double the OA (1/15). Any idea how to use the combination formula correctly in this scenario?
Thanks
S.
I'm working on my last weak area before I am going to take the GMAT: PROBABILITY AND COMBINATORICS. So I apologize in advance if some of the questions I'm going to ask you guys are too easy.
QST: In a box with 10 blocks, 3 of which are red, what is the probability of picking a red block on each of your first two tries? Assume that you do NOT replace the first block after you have picked it.
ANS: I'm aware of the traditional way to solve problems like this (probability trees). By doing so, the answer is 3/10 * 2/9 = 1/15. Pretty straight forward.
Now, I want to also be able to solve this kind of problem with a counting method (combinatorics). So I thought I can solve it like this:
(3*2)/(10C2)
Unfortunately, I get the wrong result (2/15). This is double the OA (1/15). Any idea how to use the combination formula correctly in this scenario?
Thanks
S.
Archived Topic
Hi there,
This topic has been closed and archived due to inactivity or violation of community quality standards. No more replies are possible here.
Where to now? Join ongoing discussions on thousands of quality questions in our Quantitative Questions Forum
Still interested in this question? Check out the "Best Topics" block below for a better discussion on this exact question, as well as several more related questions.
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05 Nov 2009, 07:34
Kudos
Bookmarks
enfinity
Hi there,
I'm working on my last weak area before I am going to take the GMAT: PROBABILITY AND COMBINATORICS. So I apologize in advance if some of the questions I'm going to ask you guys are too easy.
QST: In a box with 10 blocks, 3 of which are red, what is the probability of picking a red block on each of your first two tries? Assume that you do NOT replace the first block after you have picked it.
ANS: I'm aware of the traditional way to solve problems like this (probability trees). By doing so, the answer is 3/10 * 2/9 = 1/15. Pretty straight forward.
Now, I want to also be able to solve this kind of problem with a counting method (combinatorics). So I thought I can solve it like this:
(3*2)/(10C2)
Unfortunately, I get the wrong result (2/15). This is double the OA (1/15). Any idea how to use the combination formula correctly in this scenario?
Thanks
S.
I'm working on my last weak area before I am going to take the GMAT: PROBABILITY AND COMBINATORICS. So I apologize in advance if some of the questions I'm going to ask you guys are too easy.
QST: In a box with 10 blocks, 3 of which are red, what is the probability of picking a red block on each of your first two tries? Assume that you do NOT replace the first block after you have picked it.
ANS: I'm aware of the traditional way to solve problems like this (probability trees). By doing so, the answer is 3/10 * 2/9 = 1/15. Pretty straight forward.
Now, I want to also be able to solve this kind of problem with a counting method (combinatorics). So I thought I can solve it like this:
(3*2)/(10C2)
Unfortunately, I get the wrong result (2/15). This is double the OA (1/15). Any idea how to use the combination formula correctly in this scenario?
Thanks
S.
Total # of favorable outcomes: 2 red block out of three: 3C2=3
Total # of outcomes: any 2 blocks out of 10: 10C2=45
P=3/45=1/15
If you are putting in denominator the number of selections of 2 out of 10, you should also put in nominator number of selections of 2 out of 3.
OR
# of selections of any first block out of 10=10C1, # of selection of any second out of 9=9C1=9, total 10C1*9C1=90
# of selections of the first red block out of three=3C1=3, # of selections of second red block out of three=2C1=2, total 3C1*2C1=6.
P=6/90=1/15
Hope it helps.
It would be better to post the questions in PS or DS forums to get more and quick replies.
--== Message from the GMAT Club Team ==--
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This discussion does not meet community quality standards. It has been retired.
If you would like to discuss this question please re-post it in the respective forum. Thank you!
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THERE IS LIKELY A BETTER DISCUSSION OF THIS EXACT QUESTION.
This discussion does not meet community quality standards. It has been retired.
If you would like to discuss this question please re-post it in the respective forum. Thank you!
To review the GMAT Club's Forums Posting Guidelines, please follow these links: Quantitative | Verbal Please note - we may remove posts that do not follow our posting guidelines. Thank you.
03 Sep 2019, 00:52
Kudos
Bookmarks
Hello from the GMAT Club BumpBot!
Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).
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Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).
Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.
Archived Topic
Hi there,
This topic has been closed and archived due to inactivity or violation of community quality standards. No more replies are possible here.
Where to now? Join ongoing discussions on thousands of quality questions in our Quantitative Questions Forum
Still interested in this question? Check out the "Best Topics" block above for a better discussion on this exact question, as well as several more related questions.
Thank you for understanding, and happy exploring!
