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

It is currently 19 Sep 2014, 13:52

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 box contains blue and purple marbles only. What is the

  Question banks Downloads My Bookmarks Reviews Important topics  
Author Message
TAGS:
Senior Manager
Senior Manager
avatar
Status: Do and Die!!
Joined: 15 Sep 2010
Posts: 333
Followers: 1

Kudos [?]: 68 [0], given: 193

A box contains blue and purple marbles only. What is the [#permalink] New post 23 Oct 2010, 21:29
00:00
A
B
C
D
E

Difficulty:

  45% (medium)

Question Stats:

50% (01:44) correct 50% (00:34) wrong based on 10 sessions
A box contains blue and purple marbles only. What is the total number of blue marbles in the box?

(1) The probability of selecting a blue marble at random is 2/5

(2) If three purple marbles were removed from the box, the probability of selecting a purple marble at random would be 2/3
[Reveal] Spoiler: OA

_________________

I'm the Dumbest of All !!

Retired Moderator
User avatar
Joined: 02 Sep 2010
Posts: 807
Location: London
Followers: 76

Kudos [?]: 481 [0], given: 25

GMAT ToolKit User GMAT Tests User Reviews Badge
Re: probability [#permalink] New post 24 Oct 2010, 02:49
shrive555 wrote:
A box contains blue and purple marbles only. What is the total number of blue marbles in the box?

(1) The probability of selecting a blue marble at random is 2/5

(2) If three purple marbles were removed from the box, the probability of selecting a purple marble at random would be 2/3


(1) Says that 2 out of every 5 marbles is blue. Doesnt say how many marbles there are

(2) Let there be b blue and p purple marbles.
New prob = (p-3)/(b+p-3) = 2/3
3p-9=2b+2p-6
p=2b+3
Not enough to solve for p and b

(1+2) 1 also gives us the equation, b/p=2/5 or 5b=2p. The two equation combined can be solved to get the value of b

Answer is (c)
_________________

Math write-ups
1) Algebra-101 2) Sequences 3) Set combinatorics 4) 3-D geometry

My GMAT story

Get the best GMAT Prep Resources with GMAT Club Premium Membership

Expert Post
1 KUDOS received
Math Expert
User avatar
Joined: 02 Sep 2009
Posts: 23482
Followers: 3501

Kudos [?]: 26452 [1] , given: 2712

Re: probability [#permalink] New post 24 Oct 2010, 03:24
1
This post received
KUDOS
Expert's post
shrouded1 wrote:
shrive555 wrote:
A box contains blue and purple marbles only. What is the total number of blue marbles in the box?

(1) The probability of selecting a blue marble at random is 2/5

(2) If three purple marbles were removed from the box, the probability of selecting a purple marble at random would be 2/3


(1) Says that 2 out of every 5 marbles is blue. Doesnt say how many marbles there are

(2) Let there be b blue and p purple marbles.
New prob = (p-3)/(b+p-3) = 2/3
3p-9=2b+2p-6
p=2b+3
Not enough to solve for p and b

(1+2) 1 also gives us the equation, b/p=2/5 or 5b=2p. The two equation combined can be solved to get the value of b

Answer is (c)


I think the numbers must be wrong for this question.

Let the # of blue marbles be b and the value of purple marbles be p

(1) \frac{b}{b+p}=\frac{2}{5} (not b/p=2/5) --> 5b=2b+2p --> 3b=2p, insufficient to find the value of b.
(2) \frac{p-3}{b+p-3}=\frac{2}{3} --> 3p-9=2b+2p-6 --> p=2b+3, insufficient to find the value of b.

(1)+(2) 3b=2p and p=2b+3 --> 3b=2(2b+3) --> b=-6, but # of blue marbles can not be negative.

Consider the following:
From (1) the probability of selecting a purple marble is \frac{p}{b+p}=\frac{3}{5}=\frac{9}{15};
From (2) if three purple marbles were removed from the box, the probability of selecting a purple marble at random would be 2/3: \frac{p-3}{b+p-3}=\frac{2}{3}=\frac{10}{15};

\frac{9}{15}<\frac{10}{15} --> we removed 3 purple marbles and the probability of selecting a purple marble increased: this can not be true.

So there is something wrong with this question.
_________________

NEW TO MATH FORUM? PLEASE READ THIS: ALL YOU NEED FOR QUANT!!!

PLEASE READ AND FOLLOW: 11 Rules for Posting!!!

RESOURCES: [GMAT MATH BOOK]; 1. Triangles; 2. Polygons; 3. Coordinate Geometry; 4. Factorials; 5. Circles; 6. Number Theory; 7. Remainders; 8. Overlapping Sets; 9. PDF of Math Book; 10. Remainders; 11. GMAT Prep Software Analysis NEW!!!; 12. SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS) NEW!!!; 12. Tricky questions from previous years. NEW!!!;

COLLECTION OF QUESTIONS:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS ; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


What are GMAT Club Tests?
25 extra-hard Quant Tests

Get the best GMAT Prep Resources with GMAT Club Premium Membership

Retired Moderator
User avatar
Joined: 02 Sep 2010
Posts: 807
Location: London
Followers: 76

Kudos [?]: 481 [0], given: 25

GMAT ToolKit User GMAT Tests User Reviews Badge
Re: probability [#permalink] New post 24 Oct 2010, 03:49
actually to come to think of it, you don't even need to solve all this to know this question is wrong.

the probability of purples was 0.6
we remove some purples
instead of probability going down, it actually increases to 0.67

This is impossible

Bunuel wrote:
shrouded1 wrote:
shrive555 wrote:
A box contains blue and purple marbles only. What is the total number of blue marbles in the box?

(1) The probability of selecting a blue marble at random is 2/5

(2) If three purple marbles were removed from the box, the probability of selecting a purple marble at random would be 2/3


(1) Says that 2 out of every 5 marbles is blue. Doesnt say how many marbles there are

(2) Let there be b blue and p purple marbles.
New prob = (p-3)/(b+p-3) = 2/3
3p-9=2b+2p-6
p=2b+3
Not enough to solve for p and b

(1+2) 1 also gives us the equation, b/p=2/5 or 5b=2p. The two equation combined can be solved to get the value of b

Answer is (c)


I think the numbers must be wrong for this question.

Let the # of blue marbles be b and the value of purple marbles be p

(1) \frac{b}{b+p}=\frac{2}{5} (not b/p=2/5) --> 5b=2b+2p --> 3b=2p, insufficient to find the value of b.
(2) \frac{p-3}{b+p-3}=\frac{2}{3} --> 3p-9=2b+2p-6 --> p=2b+3, insufficient to find the value of b.

(1)+(2) 3b=2p and p=2b+3 --> 3b=2(2b+3) --> b=-6, but # of blue marbles can not be negative.

Consider the following:
From (1) the probability of selecting a purple marble is \frac{p}{b+p}=\frac{3}{5}=\frac{9}{15};
From (2) if three purple marbles were removed from the box, the probability of selecting a purple marble at random would be 2/3: \frac{p-3}{b+p-3}=\frac{2}{3}=\frac{10}{15};

\frac{9}{15}<\frac{10}{15} --> we removed 3 purple marbles and the probability of selecting a purple marble increased: this can not be true.

So there is something wrong with this question.

_________________

Math write-ups
1) Algebra-101 2) Sequences 3) Set combinatorics 4) 3-D geometry

My GMAT story

Get the best GMAT Prep Resources with GMAT Club Premium Membership

Manager
Manager
avatar
Status: Keep fighting!
Affiliations: IIT Madras
Joined: 31 Jul 2010
Posts: 238
WE 1: 2+ years - Programming
WE 2: 3+ years - Product developement,
WE 3: 2+ years - Program management
Followers: 4

Kudos [?]: 169 [0], given: 104

Re: probability [#permalink] New post 24 Oct 2010, 04:10
yeah ... it gives negative number of purples! What do we select as the answer if at all a question like this comes up? I selected E :)
Expert Post
Math Expert
User avatar
Joined: 02 Sep 2009
Posts: 23482
Followers: 3501

Kudos [?]: 26452 [0], given: 2712

Re: probability [#permalink] New post 24 Oct 2010, 04:14
Expert's post
Senior Manager
Senior Manager
avatar
Status: Do and Die!!
Joined: 15 Sep 2010
Posts: 333
Followers: 1

Kudos [?]: 68 [0], given: 193

Re: probability [#permalink] New post 24 Oct 2010, 11:17
DS question asks if we have sufficient information or not. in this context can't we opt for C as it gives information regardless what sign comes at the end.
_________________

I'm the Dumbest of All !!

Expert Post
Math Expert
User avatar
Joined: 02 Sep 2009
Posts: 23482
Followers: 3501

Kudos [?]: 26452 [0], given: 2712

Re: probability [#permalink] New post 24 Oct 2010, 11:27
Expert's post
shrive555 wrote:
DS question asks if we have sufficient information or not. in this context can't we opt for C as it gives information regardless what sign comes at the end.


Technically yes, we can find the value of b with (1)+(2) but again the question is flawed and there is no way you'll see such question on real test.
_________________

NEW TO MATH FORUM? PLEASE READ THIS: ALL YOU NEED FOR QUANT!!!

PLEASE READ AND FOLLOW: 11 Rules for Posting!!!

RESOURCES: [GMAT MATH BOOK]; 1. Triangles; 2. Polygons; 3. Coordinate Geometry; 4. Factorials; 5. Circles; 6. Number Theory; 7. Remainders; 8. Overlapping Sets; 9. PDF of Math Book; 10. Remainders; 11. GMAT Prep Software Analysis NEW!!!; 12. SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS) NEW!!!; 12. Tricky questions from previous years. NEW!!!;

COLLECTION OF QUESTIONS:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS ; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


What are GMAT Club Tests?
25 extra-hard Quant Tests

Get the best GMAT Prep Resources with GMAT Club Premium Membership

Manager
Manager
avatar
Joined: 24 Jul 2010
Posts: 92
Followers: 1

Kudos [?]: 29 [0], given: 12

Re: probability [#permalink] New post 25 Oct 2010, 09:02
tricky question though !
Re: probability   [#permalink] 25 Oct 2010, 09:02
    Similar topics Author Replies Last post
Similar
Topics:
Experts publish their posts in the topic A box contains only red, white and blue chips. If a chip is Fijisurf 5 15 Dec 2010, 08:11
A box contains red and blue balls only. If there are 8 balls udaymathapati 1 27 Nov 2010, 19:36
A box contains red and blue balls only. If there are 8 balls Economist 1 25 Oct 2009, 04:19
Experts publish their posts in the topic 5 blue marbles, 3 red marbles and 4 purple marbles are netcaesar 7 25 Jan 2008, 09:44
11 Experts publish their posts in the topic 5 blue marbles, 3 red marbles and 4 purple marbles are londonluddite 18 16 Oct 2006, 10:42
Display posts from previous: Sort by

A box contains blue and purple marbles only. What is the

  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®.