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

Redeem

Trivial Probability?

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
Senior | Next Rank: 100 Posts
Posts: 39
Joined: Tue Jul 05, 2011 1:13 am

Trivial Probability?

by StoneBlack » Thu Dec 29, 2011 9:13 pm
A box contains 3 yellow balls and 5 black balls. One by one, every ball is selected at random without replacement. What is the probability that the fourth ball selected is black?
A. 1/4
B. 1/2
C. 5/8
D. 2/3
E. 3/4

This question is from 300+ GMAT questions document available on BeatTheGMAT forums.
The solution is provided there but I am not really convinced of it.
Top
Source: — Problem Solving |
Legendary Member
Posts: 966
Joined: Sat Jan 02, 2010 8:06 am
Thanked: 230 times
Followed by:21 members

by shankar.ashwin » Thu Dec 29, 2011 9:31 pm
Each ball picked has a 3/8th probability of being yellow and 5/8th probability of being black. The number of selection doesn't matter.

Ofcourse theres a mathematical proof which is lengthy, but think about it, each ball is likely to be either yellow or black, now how does the number of pick matter..

If the probability was any greater or lesser you would have to say, the first 3 picks were biased towards picking a particular color, which is not the case. Hence probability at any pick shouldn't change.

P (nth pick) = P (1st pick) = [spoiler]5/8[/spoiler]
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 » Thu Dec 29, 2011 10:21 pm
Check my posts here:

https://www.beatthegmat.com/probablity-ques-t60161.html
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
Senior | Next Rank: 100 Posts
Posts: 39
Joined: Tue Jul 05, 2011 1:13 am

by StoneBlack » Thu Dec 29, 2011 11:11 pm
Thanks for the pointer.
Apparently i tried the expanded way by considering all possibilities:
3Y,1B
2Y2B
1Y3B
4B

but failed to take into consideration the different arrangement possible in 2Y2B and 1Y3B.
Thanks again!
Top
Post new topic Post Reply
• Page 1 of 1