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

Redeem
TTP OnDemand GMAT Video Course
Featured Course

Target Test Prep GMAT OnDemand

Scott Woodbury-Stewart’s private virtual classroom — 400 hours of master-class video lessons for the GMAT Focus Edition.

  • 715+ score guarantee — highest in the industry (99th percentile)
  • 52 chapters · 1,500+ lessons · 4,000+ practice questions
  • 400 hours of video · 1,500+ instructor-led HD smartboard lessons
  • 300,000+ students accepted to Harvard, Stanford, Wharton, Booth & Sloan & more
  • 24/7 live support + weekly Zoom office hours with GMAT instructors
  • TTP AI Assist — 24/7 AI-powered virtual tutor for instant help
  • 1,200+ flashcards + AI-powered study assistant & daily calendar
  • OnDemand, LiveTeach & GMAT Bootcamp formats available
  • Also: GRE, SAT Math & Executive Assessment courses
  • MBA Admissions Consulting now available
  • 🏆 2025 EdTech Breakthrough Award: Test Prep Solution Provider of the Year
  • 200,000+ students served
  • 5-day free trial — $0 to start, no auto-billing, cancel anytime
★★★★★ 5.0 (559 reviews)
130-pt guarantee
$0 to start then $127/mo
Start Free 5-Day Trial Read 559 Reviews →

Combinations

Expert replies
Post new topic Post Reply
Previous Topic Next Topic
Senior | Next Rank: 100 Posts
Posts: 43
Joined: Sat Nov 06, 2010 12:04 pm
by Woozler » Mon Dec 27, 2010 8:01 pm
If we have 5 red, 4 white, 3 black and 2 yellow marbles, how many different 2 marble combinations are there? This is a really basic combinatorics question, yet I'm blanking. What is the general formula for that? The Manhattan GMAT book was not particularly thorough with this.

Thanks all.
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Source: — Problem Solving |
Junior | Next Rank: 30 Posts
Posts: 11
Joined: Fri Dec 11, 2009 10:15 am
Thanked: 2 times

by anu009 » Mon Dec 27, 2010 8:17 pm
Hello

The combination formula goes nCr where it is equal to n!/r!(n-r)!

If you subsitute the values of picking r itmes from a set of n that is 2 marbles from a set of 4 you will get 6 as the result.
Let me know what are the answer choices and if this is the correct solution.

Thanks
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Senior | Next Rank: 100 Posts
Posts: 43
Joined: Sat Nov 06, 2010 12:04 pm

by Woozler » Mon Dec 27, 2010 8:22 pm
But why are we paying no attention to the fact that there are 4 different colors?
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY

GMAT/MBA Expert

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

by Anurag@Gurome » Mon Dec 27, 2010 8:43 pm
Woozler wrote:If we have 5 red, 4 white, 3 black and 2 yellow marbles, how many different 2 marble combinations are there?
The number of "different 2 marble combinations" is = 4C2 = 6

Why is so?
Because a red-red pair is same as another red-red pair. Also a red-black pair is same as another red-black pair. Only way we can get "different 2 marble combination" if we consider each of them of different color.

Therefore the problem reduces to: In how many ways 2 colors can be selected out of 4?
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
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Post new topic Post Reply
• Page 1 of 1