Find all School-related info fast with the new School-Specific MBA Forum

It is currently 01 Oct 2014, 06:24

Close

GMAT Club Daily Prep

Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track
Your Progress

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.

Events & Promotions

Events & Promotions in June
Open Detailed Calendar

A bag has 15 marbles , 7 black, 8 red . You pick 5 marbles

  Question banks Downloads My Bookmarks Reviews Important topics  
Author Message
TAGS:
CEO
CEO
avatar
Joined: 15 Aug 2003
Posts: 3470
Followers: 60

Kudos [?]: 670 [0], given: 781

A bag has 15 marbles , 7 black, 8 red . You pick 5 marbles [#permalink] New post 08 Oct 2003, 20:10
1
This post was
BOOKMARKED
00:00
A
B
C
D
E

Difficulty:

(N/A)

Question Stats:

0% (00:00) correct 0% (00:00) wrong based on 0 sessions
# 1.
A bag has 15 marbles , 7 black, 8 red . You pick 5 marbles one by one. The first four marbles are red.

1. Whats the probability that the fifth marble is red.
2. Whats the probability that the fifth marble is black.


# 2.
A box contains 10 balls ,7 green, 3 yellow . You pick 5 one by one. Out of the first three, 2 green and 1 yellow.

Whats the probability that the remaining balls are a GREEN and a yellow ?


Please explain.

Thanks
Praetorian

Last edited by Praetorian on 08 Oct 2003, 21:40, edited 1 time in total.
SVP
SVP
User avatar
Joined: 03 Feb 2003
Posts: 1613
Followers: 6

Kudos [?]: 47 [0], given: 0

 [#permalink] New post 08 Oct 2003, 21:10
#1
1. Whats the probability that the fifth marble is red.
8/15*7/14*6/13*5/12(four are red)*4/11 (red again)

2. Whats the probability that the fifth marble is black.
8/15*7/14*6/13*5/12(four are red)*7/11 (black)

#2
A box contains 10 balls ,7 green, 3 yellow . You pick 5 one by one. Out of the first three, 2 green and 1 yellow.

Whats the probability that the remaining balls are a red and a yellow ?

P=0 there are no red balls in a box
CEO
CEO
avatar
Joined: 15 Aug 2003
Posts: 3470
Followers: 60

Kudos [?]: 670 [0], given: 781

 [#permalink] New post 08 Oct 2003, 21:44
stolyar wrote:
#1
1. Whats the probability that the fifth marble is red.
8/15*7/14*6/13*5/12(four are red)*4/11 (red again)

2. Whats the probability that the fifth marble is black.
8/15*7/14*6/13*5/12(four are red)*7/11 (black)

#2
A box contains 10 balls ,7 green, 3 yellow . You pick 5 one by one. Out of the first three, 2 green and 1 yellow.

Whats the probability that the remaining balls are a red and a yellow ?

P=0 there are no red balls in a box


Are you sure that the probability DOES NOT change even if we have knowledge of prior events.

This problem is different in that ...we already know that some event has occured..so wont that have an impact on the probability of a future event

We want to find out P( fifth is red / four are red )

i think your solution to # 1 will be correct ONLY when we want to know whats the probability of picking 5 red balls.

again for #2, i have the same doubt.

can you clarify please?

thanks
praetorian
SVP
SVP
User avatar
Joined: 03 Feb 2003
Posts: 1613
Followers: 6

Kudos [?]: 47 [0], given: 0

 [#permalink] New post 08 Oct 2003, 22:17
You mean this approach?

P(5R)=P(4R)*P(R/4R)
5C5/12C5=5C4/12C4*X
X=[5C5/12C5]/[5C4/12C4]=1/8
CEO
CEO
avatar
Joined: 15 Aug 2003
Posts: 3470
Followers: 60

Kudos [?]: 670 [0], given: 781

 [#permalink] New post 08 Oct 2003, 22:53
stolyar wrote:
You mean this approach?

P(5R)=P(4R)*P(R/4R)
5C5/12C5=5C4/12C4*X
X=[5C5/12C5]/[5C4/12C4]=1/8



I am not quite sure about what you you did there...

I thought of using bayes theorem..but since GMAT does not test on bayes theorem, i was wondering if there was a simpler way out.

i will check bayes theorem and let you know whether 1/8 is correct.

thanks
praetorian
SVP
SVP
User avatar
Joined: 03 Feb 2003
Posts: 1613
Followers: 6

Kudos [?]: 47 [0], given: 0

 [#permalink] New post 08 Oct 2003, 23:00
I employed a basic formula for conditional probability.
Intern
Intern
avatar
Joined: 29 Aug 2003
Posts: 49
Location: Detroit, MI
Followers: 0

Kudos [?]: 2 [0], given: 0

Re: PS : Probability (conditional) [#permalink] New post 09 Oct 2003, 06:46
praetorian123 wrote:
# 1.
A bag has 15 marbles , 7 black, 8 red . You pick 5 marbles one by one. The first four marbles are red.

1. Whats the probability that the fifth marble is red.
2. Whats the probability that the fifth marble is black.


# 2.
A box contains 10 balls ,7 green, 3 yellow . You pick 5 one by one. Out of the first three, 2 green and 1 yellow.

Whats the probability that the remaining balls are a GREEN and a yellow ?


Please explain.

Thanks
Praetorian



Praet,

#1 - Since it's given that the first 4 are red,
Prob. that 5th is red = 4/11 (We have to find out the probability only for the 5th one to be red and not for all 5 to be red.)
Similarly, Prob. that 5th is black = 7/11.

#2 - I am confused as to what the question means.

Amar.
Re: PS : Probability (conditional)   [#permalink] 09 Oct 2003, 06:46
    Similar topics Author Replies Last post
Similar
Topics:
38 Experts publish their posts in the topic A bag of 10 marbles contains 3 red marbles and 7 blue bmwhype2 28 06 Dec 2007, 14:24
If you have 250 marbles, 15 are black, and you pull 10 at ethan_casta 7 21 May 2007, 15:56
A bag contains 10 marbles - 3 red, 2 green and 5 blue sumitsarkar82 7 27 Jul 2006, 00:25
Griffin has a bag of marbles that contains only 6 black faisalt 9 17 Nov 2005, 18:54
There are 8 marbles in a bag, 4 of them are red and others pb_india 7 18 May 2005, 08:49
Display posts from previous: Sort by

A bag has 15 marbles , 7 black, 8 red . You pick 5 marbles

  Question banks Downloads My Bookmarks Reviews Important topics  


GMAT Club MBA Forum Home| About| Privacy Policy| Terms and Conditions| GMAT Club Rules| Contact| Sitemap

Powered by phpBB © phpBB Group and phpBB SEO

Kindly note that the GMAT® test is a registered trademark of the Graduate Management Admission Council®, and this site has neither been reviewed nor endorsed by GMAC®.