BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem

conditional probability

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
Legendary Member
Posts: 512
Joined: Mon Jun 18, 2012 11:31 pm
Thanked: 42 times
Followed by:20 members

conditional probability

by sana.noor » Sat Mar 02, 2013 9:23 am
Denette tosses two coins. What is the probability that she has tossed 2 heads, given that she has tossed at least 1 head?
a)1/3
b)1/4
c)4/3
d)3/4
e)1/2

a
Work hard in Silence, Let Success make the noise.

If you found my Post really helpful, then don't forget to click the Thank/follow me button. :)
Top
Source: — Problem Solving |

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 16207
Joined: Mon Dec 08, 2008 6:26 pm
Location: Vancouver, BC
Thanked: 5254 times
Followed by:1268 members
GMAT Score:770

by Brent@GMATPrepNow » Sat Mar 02, 2013 9:44 am
sana.noor wrote:Denette tosses two coins. What is the probability that she has tossed 2 heads, given that she has tossed at least 1 head?
a)1/3
b)1/4
c)4/3
d)3/4
e)1/2

a
Here's a non-formulaic approach.

I'll assume that the probability of tossing heads = the probability of tossing tails = 0.5
Since each outcome is equally likely, let's draw a tree diagram to see all of the possible outcomes:
Image

So, there are 4 possible outcomes and each outcome is equally likely.
The 4 outcomes are HH, HT, TH and TT

Now it's given that there's at least one head.
So, of the 4 possible outcomes, only 3 of them are such that there's at least one head.
Image

Now that we know we've narrowed the field to three equally-likely outcomes (HH, HT and TH), we want the probability of HH
Image

The probability is [spoiler]1/3 (answer = A)[/spoiler]

Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
Image
Top
Legendary Member
Posts: 512
Joined: Mon Jun 18, 2012 11:31 pm
Thanked: 42 times
Followed by:20 members

by sana.noor » Sat Mar 02, 2013 10:22 am
Brent should one practice Binomal theorem to find the probability? is it important to study for gmat or one can skip?
Work hard in Silence, Let Success make the noise.

If you found my Post really helpful, then don't forget to click the Thank/follow me button. :)
Top

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 16207
Joined: Mon Dec 08, 2008 6:26 pm
Location: Vancouver, BC
Thanked: 5254 times
Followed by:1268 members
GMAT Score:770

by Brent@GMATPrepNow » Sat Mar 02, 2013 10:24 am
Well, you could use the binomial theorem for some questions, but the GMAT would never ask you a question that requires it.
I'd skip it.

Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
Image
Top
User avatar
GMAT Instructor
Posts: 15539
Joined: Tue May 25, 2010 12:04 pm
Location: New York, NY
Thanked: 13060 times
Followed by:1906 members
GMAT Score:790

by GMATGuruNY » Sat Mar 02, 2013 1:54 pm
sana.noor wrote:Denette tosses two coins. What is the probability that she has tossed 2 heads, given that she has tossed at least 1 head?
a)1/3
b)1/4
c)4/3
d)3/4
e)1/2

a
Here are all of ways to get at least one heads:
HT
HH
TH

Of these 3 options, one the outcome in red is favorable, implying that the probability = 1/3.

The correct answer is A.
Private tutor exclusively for the GMAT and GRE, with over 20 years of experience.
Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.

As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.

For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3
Top

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 3835
Joined: Fri Apr 02, 2010 10:00 pm
Location: Milpitas, CA
Thanked: 1854 times
Followed by:523 members
GMAT Score:770

by Anurag@Gurome » Sat Mar 02, 2013 7:28 pm
sana.noor wrote:Denette tosses two coins. What is the probability that she has tossed 2 heads, given that she has tossed at least 1 head?
Number of outcomes with at least one head = 3 {HH, TH, HT}

Number of outcomes with two heads = 1 {HH}

Hence, required probability = 1/3

The correct answer is A.
Anurag Mairal, Ph.D., MBA
GMAT Expert, Admissions and Career Guidance
Gurome, Inc.
1-800-566-4043 (USA)

Join Our Facebook Groups
GMAT with Gurome
https://www.facebook.com/groups/272466352793633/
Admissions with Gurome
https://www.facebook.com/groups/461459690536574/
Career Advising with Gurome
https://www.facebook.com/groups/360435787349781/
Top
Post new topic Post Reply
• Page 1 of 1