Find all School-related info fast with the new School-Specific MBA Forum

It is currently 27 Aug 2015, 18:32
GMAT Club Tests

Close

GMAT Club Daily Prep

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.

Events & Promotions

Events & Promotions in June
Open Detailed Calendar

A certain jewelry store sells customized rings in which

  Question banks Downloads My Bookmarks Reviews Important topics  
Author Message
Manager
Manager
avatar
Joined: 02 Jul 2004
Posts: 51
Followers: 0

Kudos [?]: 2 [0], given: 0

A certain jewelry store sells customized rings in which  [#permalink] New post 15 Oct 2004, 12:21
1
This post was
BOOKMARKED
00:00
A
B
C
D
E

Difficulty:

(N/A)

Question Stats:

0% (00:00) correct 0% (00:00) wrong based on 0 sessions
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

A certain jewelry store sells customized rings in which
three gemstones selected by the customer are set in
a straight row along the band of the ring. If exactly
5 different types of gemstones are available, and if
at least two of the gemstones in any given ring must
be different, how many different rings are possible?

(a) 20
(b) 60
(c) 90
(d) 120
(e) 210
Senior Manager
Senior Manager
avatar
Joined: 19 May 2004
Posts: 291
Followers: 1

Kudos [?]: 11 [0], given: 0

 [#permalink] New post 15 Oct 2004, 13:18
Total ways: 5^3 = 125.
Unwanted: 5. (All the same)
125 - 5 = 120
Manager
Manager
avatar
Joined: 02 Jul 2004
Posts: 51
Followers: 0

Kudos [?]: 2 [0], given: 0

 [#permalink] New post 15 Oct 2004, 13:26
Thats correct ...
OA is D

When I went to work the problem I kept trying to do

5 possibilities for 1st stone
5 possibilities for 2nd stone
4 possibilities for 3rd stone

or 5*5*4 = 100.

I can see that this is not quite right (for one reason need to divide out identical cases), however can someone please explain why this method is wrong and, if possible, what it's trying to calculate?

Thanks
Senior Manager
Senior Manager
avatar
Joined: 19 May 2004
Posts: 291
Followers: 1

Kudos [?]: 11 [0], given: 0

 [#permalink] New post 15 Oct 2004, 16:24
tyagel, It is more complicated this way. You didn't count all the options.

There are 2 options:
1) All different = 5*4*3 = 60
+
2) 2 Equal, 1 Different = 5C2 * 2 * 3C2 = 60.

60 + 60 = 120.

Let me explain the second one:
5C2 : Pick 2 stones out of 5 ,
2 : Choose 1 of those two colors again.
3C2 : Pick two places for the identical colors, order doesn't matter.
Manager
Manager
avatar
Joined: 18 Sep 2004
Posts: 155
Location: Dallas, TX
Followers: 1

Kudos [?]: 5 [0], given: 0

 [#permalink] New post 15 Oct 2004, 23:08
# all 3 different 5P3 = 5*4*3 = 60

# only 2 are same and 1 different (think of the two same as one) 4P2 = 12

There are 5 different stones that can be group together to form 2 same = 5 * 12

60 + (5*12) = 60+60 120 ways.
Manager
Manager
avatar
Joined: 02 Jul 2004
Posts: 51
Followers: 0

Kudos [?]: 2 [0], given: 0

 [#permalink] New post 16 Oct 2004, 06:17
Ok now I understand ...

Thanks for the help!

A slightly different way (although pretty much same a Dookie's) is

5P2*3C2

The first is a permutation since order can be considered to determine which color gets two.
  [#permalink] 16 Oct 2004, 06:17
Display posts from previous: Sort by

A certain jewelry store sells customized rings in which

  Question banks Downloads My Bookmarks Reviews Important topics  


GMAT Club MBA Forum Home| About| Privacy Policy| Terms and Conditions| GMAT Club Rules| Contact| Sitemap

Powered by phpBB © phpBB Group and phpBB SEO

Kindly note that the GMAT® test is a registered trademark of the Graduate Management Admission Council®, and this site has neither been reviewed nor endorsed by GMAC®.