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 Your Progress

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

Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.

It appears that you are browsing the GMAT Club forum unregistered!

Signing up is free, quick, and confidential.
Join other 350,000 members and get the full benefits of GMAT Club

Registration gives you:

Tests

Take 11 tests and quizzes from GMAT Club and leading GMAT prep companies such as Manhattan GMAT,
Knewton, and others. All are free for GMAT Club members.

Applicant Stats

View detailed applicant stats such as GPA, GMAT score, work experience, location, application
status, and more

Books/Downloads

Download thousands of study notes,
question collections, GMAT Club’s
Grammar and Math books.
All are free!

Thank you for using the timer!
We noticed you are actually not timing your practice. Click the START button first next time you use the timer.
There are many benefits to timing your practice, including:

Re: Probability socks [#permalink]
12 Nov 2009, 08:54

8

This post received KUDOS

Expert's post

3

This post was BOOKMARKED

Economist wrote:

A drawer contains 8 socks, and 2 socks are selected at random without replacement. What is the probability that both socks are black?

(1) The probability is less than 0.2 that the first sock is black.

(2) The probability is more than 0.8 that the first sock is white.

I suppose 8 socks means, that there are 8 and not 8 pairs.

Note that we don't know if there is any # of black socks is the drawer. Let B be the # of black socks. And W the # of white socks.

(1) \(\frac{B}{8}<0.2\) --> \(B<1.6\), so there can be 1 or 0 black socks in the drawer. In any case as the # is less then 2 the probability of picking 2 black socks is 0. Sufficient.

(2) \(\frac{W}{8}>0.8\) --> \(W>6.4\), so there are 7 or 8 white socks in the drawer. As the maximum possible # of black socks is 1, thus the probability of 2 blacks is 0. Sufficient.

Re: Probability socks [#permalink]
14 Sep 2010, 15:03

I had initially thought answer as D but later changed to A(my mistake). My thought process was that with second choice I don't know if remaining sock is black or not. I failed to calculate that the answer of two draws would be zero in this case also _________________

If you like my post, consider giving me some KUDOS !!!!! Like you I need them

Re: Black and White Socks [#permalink]
27 Nov 2010, 20:49

Expert's post

udaymathapati wrote:

A drawer contains 8 socks, and 2 socks are selected at random without replacement. What is the probability that both socks are black? (1) The probability is less than 0.2 that the first sock is black. (2) The probability is more than 0.8 that the first sock is white.

Please explain in detail.

How about I tell you on what lines to think and perhaps you can arrive at the answer? Let's say there are x black socks in the drawer. When I pick the first one, the probability of picking a black sock is x/8 which is less than 1/5. Any ideas? _________________

Re: A drawer contains 8 socks, and 2 socks are selected at [#permalink]
25 Jun 2014, 04:06

Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email. _________________

Re: A drawer contains 8 socks, and 2 socks are selected at [#permalink]
27 Jul 2014, 05:19

Hi Bunuel, Why is that in this problem the socks are assumed to be either white or black? No where in the question is it mentioned that the socks are either white or black.

Re: A drawer contains 8 socks, and 2 socks are selected at [#permalink]
27 Jul 2014, 14:51

Expert's post

dilipnair27 wrote:

Hi Bunuel, Why is that in this problem the socks are assumed to be either white or black? No where in the question is it mentioned that the socks are either white or black.

So, shouldn't the answer be E?

We are not assuming that.

From (1) we have that there could be 1 or 0 black socks in the drawer (for example, there could be 1 black and 7 red socks, or all red socks). No, matter which it is, the probability of picking 2 black socks is 0.

From (2) we have that there could be 7 or 8 white socks in the drawer (for example, there could be 7 white and 1 green, or 7 white and 1 black, or 8 white socks). So, there could be at most 1 black sock (0 or 1). So, in any case the probability of picking 2 black socks is 0.

The bottom line is that, from each statement we have that the maximum number of black socks is 1, which makes the probability of picking 2 black socks equal 0.

Type of Visa: You will be applying for a Non-Immigrant F-1 (Student) US Visa. Applying for a Visa: Create an account on: https://cgifederal.secure.force.com/?language=Englishcountry=India Complete...