Find all School-related info fast with the new School-Specific MBA Forum
Learn more about the recent changes... close

It is currently 23 May 2013, 19:53
Customize  |  Hide

Search
Register
New posts
Unanswered

Probability theory

  Question banks Downloads My Bookmarks Reviews  
  Oldest Best Reply
Search for:
 
Print view
First unread post
Author Message
TAGS:
Manager
Manager
Joined: 11 Aug 2012
Posts: 79
Followers: 0

Kudos [?]: 4 [0], given: 11

Probability theory [#permalink] New post 18 Jan 2013, 07:37
In probability questions, specifically OR problems (addition problems), the formula is:

P(A or B) = P(A) + P(B) - P(A AND B)

Why do we have to substract P(A AND B)?, I think we do that because P(A) and P(B) already include P(A AND B), right?

Please, confirm.
BSchool Thread Master
Joined: 23 Mar 2011
Posts: 591
Concentration: Healthcare, Strategy
GMAT 1: 720 Q51 V35
GMAT 2: 770 Q51 V44
WE: Consulting (Health Care)
Followers: 30

Kudos [?]: 248 [0], given: 239

Re: Probability theory [#permalink] New post 18 Jan 2013, 08:39
danzig wrote:
In probability questions, specifically OR problems (addition problems), the formula is:

P(A or B) = P(A) + P(B) - P(A AND B)

Why do we have to substract P(A AND B)?, I think we do that because P(A) and P(B) already include P(A AND B), right?

Please, confirm.


correct.
p(a) and p(b) both include p(a & b). so essentially when you want to write p(a or b), you mean p(only a)+p(only b)+p(a&b). ---1

where as, p(a)+p(b)= [p(only a)+ p(a&b)] + [p(only b)+p(a&b)]

hence you want to subtract p(a&b) from p(a)+p(b) to obtain equation #1.

Hope its clear. You should also try making a Venn diagram; that shall make this a bit more easier.
_________________

My Debrief | Find out what's new at GMAT Club

Veritas Prep GMAT Instructor
User avatar
Joined: 16 Oct 2010
Posts: 3111
Location: Pune, India
Followers: 572

Kudos [?]: 2012 [0], given: 92


Top Member of the Month
Re: Probability theory [#permalink] New post 21 Jan 2013, 06:04
danzig wrote:
In probability questions, specifically OR problems (addition problems), the formula is:

P(A or B) = P(A) + P(B) - P(A AND B)

Why do we have to substract P(A AND B)?, I think we do that because P(A) and P(B) already include P(A AND B), right?

Please, confirm.


Yes, it is the same as the sets concept:

n(A or B) = n(A) + n(B) - n(A and B)

n(A) and n(B) both include n(A and B). You cannot double count it so you must subtract it once. Check out this sets post to see how it is represented in the venn diagram:
http://www.veritasprep.com/blog/2012/09 ... s-of-sets/

If you need to find the probability of 'A or B but not both', you will need to subtract P(A and B) twice.

P(A or B but not both) = P(A) + P(B) - 2*P(A and B)
_________________

Karishma
Veritas Prep | GMAT Instructor
My Blog

Save 10% on Veritas Prep GMAT Courses And Admissions Consulting
Enroll now. Pay later. Take advantage of Veritas Prep's flexible payment plan options.

Veritas Prep Reviews

Re: Probability theory   [#permalink] 21 Jan 2013, 06:04
    Similar topics Author Replies Last post
Similar
Topics:
 New posts Probability gmatblast 7 11 Feb 2004, 13:57
New posts Probability boksana 3 14 May 2004, 18:20
New posts Theory&Tips for probability andrehaui 1 19 Apr 2007, 05:26
New posts 1 in theory blog 3 23 Jan 2008, 21:34
New posts 2 Probability Theory & Statistics Fundamental New Doc vivgmat 2 18 Aug 2011, 13:57
Display posts from previous: Sort by

Probability theory

  Question banks Downloads My Bookmarks Reviews  
Print view
First unread post

Moderators: Bunuel, VeritasPrepKarishma

Search for:


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®.

Main navigation

GMAT Resources

Partners

MBA Resources

www.gmac.com|www.mba.com