gmat preparation courses

It is currently Tue Jun 18, 2019 6:01 pm

All times are UTC - 5 hours [ DST ]




Post new topic Reply to topic  [ 2 posts ] 
Author Message
 Post subject: GRE Probability
PostPosted: Sun Dec 23, 2012 4:22 am 
Offline

Joined: Tue Jan 15, 2013 3:59 am
Posts: 2
Event A occurs with probability 0.3. Event B occurs with probability 0.5. If A and B are independent, what is the probability that event A or event B occurs?

A. 0.15
B. 0.20
C. 0.65
D. 0.80
E. 0.92

The correct answer is (C).

In order to find the probability that A or B occurs, we use the formula P(A or B) = P(A) + P(B) – P(A and B). Remember that to find P(A and B) , we only have to multiply the two probabilities together, since A and B are independent events.

Substituting, we get P(A or B) = 0.3 + 0.5 – (0.3)(0.5) = 0.3 + 0.5 – 0.15. This becomes 0.8 – 0.15, which is 0.65, choice (C).

Note that if we were to simply add P(A) + P(B) to get an answer of 0.8, we would have double-counted the event when A and B occur at the same time. That is why we had to subtract their overlap once, to get rid of the double-counted overlap.
----------
It states that the events are independent and nowhere does it state that they can occur together so why would you subtract the (.3)(.5) after you have .8 ?


Top
 Profile  
 
 Post subject: Re: GRE Probability
PostPosted: Sun Dec 23, 2012 4:54 am 
Offline

Joined: Sun May 30, 2010 2:23 am
Posts: 498


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 2 posts ] 

All times are UTC - 5 hours [ DST ]


Who is online

Users browsing this forum: No registered users and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
cron
Powered by phpBB © 2000, 2002, 2005, 2007 phpBB Group