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

 It is currently 04 May 2015, 01:24

### 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

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

# Events & Promotions

###### Events & Promotions in June
Open Detailed Calendar

# 3) There are 12 white balls and 12 black balls that are

Author Message
TAGS:
CEO
Joined: 15 Aug 2003
Posts: 3469
Followers: 61

Kudos [?]: 702 [0], given: 781

3) There are 12 white balls and 12 black balls that are [#permalink]  12 Oct 2003, 20:13
00:00

Difficulty:

(N/A)

Question Stats:

0% (00:00) correct 0% (00:00) wrong based on 0 sessions
3) There are 12 white balls and 12 black balls that are dispersed
randomly. If the first 3 balls are black, what is the probability
that fourth is also black.
SVP
Joined: 03 Feb 2003
Posts: 1608
Followers: 6

Kudos [?]: 76 [0], given: 0

P (4 black) = P (3 black) * P (1b/3b)

P (4 black) = 12C4/24C4
P (3 black) = 12C3/24C3

P (1b/3b)=(12C4/24C4)/(12C3/24C3)=9/21=3/7
CEO
Joined: 15 Aug 2003
Posts: 3469
Followers: 61

Kudos [?]: 702 [0], given: 781

stolyar wrote:
P (4 black) = P (3 black) * P (1b/3b)

P (4 black) = 12C4/24C4
P (3 black) = 12C3/24C3

P (1b/3b)=(12C4/24C4)/(12C3/24C3)=9/21=3/7

Stolyar

Known that first three balls are black, so now we have 12 white and 9

black.

Then you pick the next ball, it being black will have a probability of

9/21 = 3/7

That begs the question, is my thinking right?

thanks
praetorian
SVP
Joined: 03 Feb 2003
Posts: 1608
Followers: 6

Kudos [?]: 76 [0], given: 0

I think yes, for the basic formula you employ is affected by the fact that 3 black ones are out.
Similar topics Replies Last post
Similar
Topics:
7 Given a bag with 2 black balls and 3 white balls 9 18 Jan 2012, 02:23
23 A bag contains 3 white balls, 3 black balls & 2 red balls 19 29 Aug 2010, 08:06
2 A bag contains 3 white balls, 3 black balls & 2 red balls. 13 10 Aug 2010, 16:41
there are 12 blue balls and y yellows balls. what is the 3 04 Nov 2007, 20:30
Q1. A bag contains 3 red,6 white n 7 black balls.2 balls are 4 01 Jun 2006, 10:25
Display posts from previous: Sort by