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 →

Logic of selecting items from a group

Expert replies
Post new topic Post Reply
Previous Topic Next Topic
Legendary Member
Posts: 518
Joined: Tue May 12, 2015 8:25 pm
Thanked: 10 times
by nikhilgmat31 » Wed Jul 29, 2015 10:12 pm
Please suggest when to use nCr formula to select a item for a group.

lets says we need to select 1 blue ball from 5 blue,4 Green & 3 Yellow Balls. - then we can simple take 5/12
Then What is the need of writing 5C1/12 - however 5C1 also gives 5

In other case

lets says we need to select 2 blue ball from 5 blue,4 Green & 3 Yellow Balls. - then we can simple take 5/12 * 4/11 = 5/33

In that case do we need to write it as 5C2 = 10

What will be denominator in this case-- is it 12

so probability is 5C2/12 = 10/12 = 5/6

Please suggest . I am confused.
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Source: — Problem Solving |
User avatar
Master | Next Rank: 500 Posts
Posts: 410
Joined: Fri Mar 13, 2015 3:36 amFollowed by: 8 members
Location: Worldwide
Thanked: 120 times
GMAT Score:770

by OptimusPrep » Thu Jul 30, 2015 9:01 am
Nikhil, simply take it this way.

nCr is a selection of n things from a total of r things.

For your problems,

1. Select 1 blue ball from 5 blue,4 Green & 3 Yellow Balls
This means. 1 blue ball out of 5 blue balls
and 1 ball out of total 12 balls.

Hence we write 5C1/12C1

2. Select 2 blue ball from 5 blue,4 Green & 3 Yellow Balls
This means. 2 blue ball out of 5 blue balls
and 2 balls out of total 12 balls.

So, we write 5C2/12C2 and on solving this is the same as what you have written.

What you are doing is:(5/12)*(4/11)

It simply means:
(5C1/12C1)*(4C1/11C1) = (1 blue balls from 5 blue balls and total 12 balls)*(1 blue ball from 4 blue balls and total 11 balls). Because the one ball has already been removed.

Does this help?
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Senior | Next Rank: 100 Posts
Posts: 51
Joined: Sun Jul 12, 2015 7:53 am
Thanked: 4 times

by Sun Light » Thu Jul 30, 2015 10:15 am
nikhilgmat31 wrote:Please suggest when to use nCr formula to select a item for a group.

lets says we need to select 1 blue ball from 5 blue,4 Green & 3 Yellow Balls. - then we can simple take 5/12
Then What is the need of writing 5C1/12 - however 5C1 also gives 5

In other case

lets says we need to select 2 blue ball from 5 blue,4 Green & 3 Yellow Balls. - then we can simple take 5/12 * 4/11 = 5/33

In that case do we need to write it as 5C2 = 10

What will be denominator in this case-- is it 12

so probability is 5C2/12 = 10/12 = 5/6

Please suggest . I am confused.

You are messing up combination and probability together. Both these concepts are used together is solving probability questions.

Post questions related to combination and probability ... that would be a good point to start.
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Legendary Member
Posts: 518
Joined: Tue May 12, 2015 8:25 pm
Thanked: 10 times

by nikhilgmat31 » Thu Jul 30, 2015 9:38 pm
solving question is another part of game. I want to be rich in concepts of counting, probability, combinations & permutations.

I am mixing all these 4.
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Legendary Member
Posts: 518
Joined: Tue May 12, 2015 8:25 pm
Thanked: 10 times

by nikhilgmat31 » Thu Jul 30, 2015 11:23 pm
OptimusPrep wrote:Nikhil, simply take it this way.

nCr is a selection of n things from a total of r things.

For your problems,

1. Select 1 blue ball from 5 blue,4 Green & 3 Yellow Balls
This means. 1 blue ball out of 5 blue balls
and 1 ball out of total 12 balls.

Hence we write 5C1/12C1

2. Select 2 blue ball from 5 blue,4 Green & 3 Yellow Balls
This means. 2 blue ball out of 5 blue balls
and 2 balls out of total 12 balls.

So, we write 5C2/12C2 and on solving this is the same as what you have written.

What you are doing is:(5/12)*(4/11)

It simply means:
(5C1/12C1)*(4C1/11C1) = (1 blue balls from 5 blue balls and total 12 balls)*(1 blue ball from 4 blue balls and total 11 balls). Because the one ball has already been removed.

Does this help?
Awesome explanation Ankur. Thanks so much.

Can you please provide some reference to guide to counting(combinations & permutations) & probability


Thanks in Advance.
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Post new topic Post Reply
• Page 1 of 1