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

 It is currently 04 Jul 2015, 08:45

### 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

# A box contains five red and six white balls. If four balls

 Question banks Downloads My Bookmarks Reviews Important topics
Author Message
TAGS:
Manager
Joined: 24 Jun 2003
Posts: 91
Location: Moscow
Followers: 1

Kudos [?]: 1 [0], given: 0

A box contains five red and six white balls. If four balls [#permalink]  08 Aug 2003, 02:04
A box contains five red and six white balls. If four balls are taken out of the box, find the probability that:

1) two are read and two are white;
2)all are the same color;
3) at least three are white;
Manager
Joined: 10 Jun 2003
Posts: 210
Location: Maryland
Followers: 2

Kudos [?]: 5 [0], given: 0

A) (5C2 * 6C2)/11C4 = 5/22
B) (5C4 + 6C4)/11C4 = 1/33
C) ((6C3 * 5C1) + 6C4)/11C4 = 23/132
Director
Joined: 03 Jul 2003
Posts: 654
Followers: 2

Kudos [?]: 32 [0], given: 0

[#permalink]  08 Aug 2003, 14:13
For the first one, I know this is the correct answer, but
I don't know how it should be.

I calculated P = 5/11 * 4/10 * 6/9 * 5/8

What am I missing?
GMAT Instructor
Joined: 07 Jul 2003
Posts: 770
Location: New York NY 10024
Schools: Haas, MFE; Anderson, MBA; USC, MSEE
Followers: 13

Kudos [?]: 70 [0], given: 0

[#permalink]  08 Aug 2003, 15:17
For the first one, I know this is the correct answer, but
I don't know how it should be.

I calculated P = 5/11 * 4/10 * 6/9 * 5/8

What am I missing?

First of all 5c2 * 6c2 / 11c4 = 10 * 15 / 330 = 5/11 not 5/22.

You are calculating the probability of just one way of combining 2 whites and 2 reds. If you multiply your solution by the number of ways 2W and 2R can combine (or 4c2) you will get the correct answer.
_________________

Best,

AkamaiBrah
Former Senior Instructor, Manhattan GMAT and VeritasPrep
Vice President, Midtown NYC Investment Bank, Structured Finance IT
MFE, Haas School of Business, UC Berkeley, Class of 2005
MBA, Anderson School of Management, UCLA, Class of 1993

Director
Joined: 03 Jul 2003
Posts: 654
Followers: 2

Kudos [?]: 32 [0], given: 0

[#permalink]  08 Aug 2003, 19:04
AkamaiBrah wrote:
For the first one, I know this is the correct answer, but
I don't know how it should be.

I calculated P = 5/11 * 4/10 * 6/9 * 5/8

What am I missing?

First of all 5c2 * 6c2 / 11c4 = 10 * 15 / 330 = 5/11 not 5/22.

You are calculating the probability of just one way of combining 2 whites and 2 reds. If you multiply your solution by the number of ways 2W and 2R can combine (or 4c2) you will get the correct answer.

Thank you for your response. The trouble with me is either I get it
completely or not. In this case, I've not got it.

Let us assume that we are taking 4 white balls out of 5 white and 6 red
balls then we say P = 5/11 * 4/10 * 3/9 * 2/8.
Why we need to multiply in with 4C2 if we are choosing more than one
color. Why does the order of picking the balls matter?
GMAT Instructor
Joined: 07 Jul 2003
Posts: 770
Location: New York NY 10024
Schools: Haas, MFE; Anderson, MBA; USC, MSEE
Followers: 13

Kudos [?]: 70 [0], given: 0

[#permalink]  08 Aug 2003, 19:39
AkamaiBrah wrote:
For the first one, I know this is the correct answer, but
I don't know how it should be.

I calculated P = 5/11 * 4/10 * 6/9 * 5/8

What am I missing?

First of all 5c2 * 6c2 / 11c4 = 10 * 15 / 330 = 5/11 not 5/22.

You are calculating the probability of just one way of combining 2 whites and 2 reds. If you multiply your solution by the number of ways 2W and 2R can combine (or 4c2) you will get the correct answer.

Thank you for your response. The trouble with me is either I get it
completely or not. In this case, I've not got it.

Let us assume that we are taking 4 white balls out of 5 white and 6 red
balls then we say P = 5/11 * 4/10 * 3/9 * 2/8.
Why we need to multiply in with 4C2 if we are choosing more than one
color. Why does the order of picking the balls matter?

It doesn't matter. But you are calculating the SPECIFIC probability that the ball will come up in a specific sequence (2R then 2W). Hence, you are missing the other five ways.
_________________

Best,

AkamaiBrah
Former Senior Instructor, Manhattan GMAT and VeritasPrep
Vice President, Midtown NYC Investment Bank, Structured Finance IT
MFE, Haas School of Business, UC Berkeley, Class of 2005
MBA, Anderson School of Management, UCLA, Class of 1993

[#permalink] 08 Aug 2003, 19:39
Similar topics Replies Last post
Similar
Topics:
A box contains 100 balls each numbered 1 to 100. 3 balls are 3 31 May 2008, 03:30
One box contains 30 purple balls and 20 red balls, another 1 25 Aug 2007, 23:35
1 A box contains 100 balls numbered from 1 to 100. If 3 balls 13 09 Mar 2007, 01:56
1 25 balls in a box. colors: red, white or blue. each numbered 1 01 Feb 2007, 04:11
Each of 25 balls in a box are either blue, red, white and 8 10 Oct 2005, 08:10
Display posts from previous: Sort by

# A box contains five red and six white balls. If four balls

 Question banks Downloads My Bookmarks Reviews Important topics

 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®.