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 →

From a group of 21 people, how many different 3-member crews

Expert replies
Post new topic Post Reply
Previous Topic Next Topic
Senior | Next Rank: 100 Posts
Posts: 34
Joined: Sun Aug 29, 2010 11:48 am
Thanked: 3 times
by Adi_Pat » Tue Sep 07, 2010 10:11 am
From a group of 21 people - out of whom 12 have experience - a 3 person crew is selected such that only 1 has experience. How many different crews can be selected ?

1) 432
2) 594
3) 864
4) 1330
5) 7980

OA A
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: 422
Joined: Mon Aug 09, 2010 11:47 amFollowed by: 1 member
Thanked: 22 times
GMAT Score:680

by beatthegmatinsept » Tue Sep 07, 2010 10:45 am
Adi_Pat wrote:From a group of 21 people - out of whom 12 have experience - a 3 person crew is selected such that only 1 has experience. How many different crews can be selected ?

1) 432
2) 594
3) 864
4) 1330
5) 7980

OA A
I am usually not great with Combination problems, but I'll try explaining this one.

We need to select 1 person out of 12 experienced ppl. So 12C1 ways to select = 12 ways to select that 1 person.
We need to select 2 people out of 9 non-experienced ppl. So 9C2 ways to select = 36 ways to select those 2 people.
Total ways = 12 * 36 = 432. (A)
Being defeated is often only a temporary condition. Giving up is what makes it permanent.
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Senior | Next Rank: 100 Posts
Posts: 34
Joined: Sun Aug 29, 2010 11:48 am
Thanked: 3 times

by Adi_Pat » Tue Sep 07, 2010 11:07 am
Makes sense with the formula...Why is this approach giving me the wrong answer...

Number of ways of selecting 1 experienced person = 12

So that leaves 9 people....

Number of ways of selecting 2nd person = 9
Number of ways of selecting 3rd person = 8

Total number of ways = 12*9*8 = 864
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
User avatar
Master | Next Rank: 500 Posts
Posts: 422
Joined: Mon Aug 09, 2010 11:47 amFollowed by: 1 member
Thanked: 22 times
GMAT Score:680

by beatthegmatinsept » Tue Sep 07, 2010 12:21 pm
Adi_Pat wrote:Makes sense with the formula...Why is this approach giving me the wrong answer...

Number of ways of selecting 1 experienced person = 12

So that leaves 9 people....

Number of ways of selecting 2nd person = 9
Number of ways of selecting 3rd person = 8

Total number of ways = 12*9*8 = 864
Nowhere in the question does it specify that the 2 inexperienced people are picked one by one, which is I believe what your approach is assuming.
Being defeated is often only a temporary condition. Giving up is what makes it permanent.
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
User avatar
GMAT Instructor
Posts: 3225
Joined: Tue Jan 08, 2008 2:40 pmFollowed by: 614 members
Location: Toronto
Thanked: 1710 times
GMAT Score:800

by Stuart@KaplanGMAT » Tue Sep 07, 2010 1:16 pm
Adi_Pat wrote:Makes sense with the formula...Why is this approach giving me the wrong answer...

Number of ways of selecting 1 experienced person = 12

So that leaves 9 people....

Number of ways of selecting 2nd person = 9
Number of ways of selecting 3rd person = 8

Total number of ways = 12*9*8 = 864
Hi! You've worked out the problem as though order matters, which is why you got 72 instead of 36 for the second step (inexperienced) part of the question.

If the question had read:
3 seats in a canoe are going to be filled by 1 experienced person and 2 inexperienced people. There are 12 experienced canoeists and 9 inexperienced canoeists available. If the experienced canoeist must sit in the back and the inexperienced canoeists will occupy the first two seats, in how many ways can the 3 seats be filled?
then your solution would have been perfect. However, in the actual question we don't care about order, so you've double counted.

Let's call the inexperienced people A, B, C, D, E, F, G, H and I.

The way you've solved the problem, you could select A first and then B second; however, you could also select B first and then A second. You've counted these as 2 distinct selections, even though they're both simply A + B selected.

So, you need to divide by 2 to eliminate the double counting.

12 * (9 * 8)/2 = 12 * 72/2 = 12 * 36 = 432
Image

Stuart Kovinsky | Kaplan GMAT Faculty | Toronto

Kaplan Exclusive: The Official Test Day Experience | Ready to Take a Free Practice Test? | Kaplan/Beat the GMAT Member Discount
BTG100 for $100 off a full course
Top
Join the discussion
Log in to postLogin with Googleuse password
QUOTE IN REPLY
Post new topic Post Reply
• Page 1 of 1