A box contains blue and purple marbles only. What is the : GMAT Data Sufficiency (DS)
Check GMAT Club App Tracker for the Latest School Decision Releases http://gmatclub.com/AppTrack

 It is currently 02 Dec 2016, 16:38

### 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

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

# Events & Promotions

###### Events & Promotions in June
Open Detailed Calendar

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

Author Message
TAGS:

### Hide Tags

Senior Manager
Status: Do and Die!!
Joined: 15 Sep 2010
Posts: 326
Followers: 1

Kudos [?]: 412 [0], given: 193

A box contains blue and purple marbles only. What is the [#permalink]

### Show Tags

23 Oct 2010, 21:29
00:00

Difficulty:

45% (medium)

Question Stats:

61% (01:55) correct 39% (01:01) wrong based on 53 sessions

### HideShow timer Statistics

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
Joined: 02 Sep 2010
Posts: 805
Location: London
Followers: 104

Kudos [?]: 936 [0], given: 25

### Show Tags

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

_________________
Math Expert
Joined: 02 Sep 2009
Posts: 35837
Followers: 6827

Kudos [?]: 89647 [1] , given: 10380

### Show Tags

24 Oct 2010, 03:24
1
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

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.
_________________
Retired Moderator
Joined: 02 Sep 2010
Posts: 805
Location: London
Followers: 104

Kudos [?]: 936 [0], given: 25

### Show Tags

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

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.

_________________
Manager
Status: Keep fighting!
Joined: 31 Jul 2010
Posts: 235
WE 1: 2+ years - Programming
WE 2: 3+ years - Product developement,
WE 3: 2+ years - Program management
Followers: 5

Kudos [?]: 426 [0], given: 104

### Show Tags

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
Math Expert
Joined: 02 Sep 2009
Posts: 35837
Followers: 6827

Kudos [?]: 89647 [0], given: 10380

### Show Tags

24 Oct 2010, 04:14
hemanthp wrote:
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

You won't see such question on GMAT.
_________________
Senior Manager
Status: Do and Die!!
Joined: 15 Sep 2010
Posts: 326
Followers: 1

Kudos [?]: 412 [0], given: 193

### Show Tags

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 !!

Math Expert
Joined: 02 Sep 2009
Posts: 35837
Followers: 6827

Kudos [?]: 89647 [0], given: 10380

### Show Tags

24 Oct 2010, 11:27
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.
_________________
Manager
Joined: 24 Jul 2010
Posts: 90
Followers: 3

Kudos [?]: 188 [0], given: 12

### Show Tags

25 Oct 2010, 09:02
tricky question though !
Re: probability   [#permalink] 25 Oct 2010, 09:02
Similar topics Replies Last post
Similar
Topics:
If there are only red, blue, and green marbles in a jar, what is the 2 11 Aug 2016, 05:58
18 A certain box contains only blue (b), green (g) and red(r) 8 26 May 2013, 09:45
8 A box contains only red chips, white chips, and blue chips. 4 14 Dec 2012, 04:19
1 A box contains only red, white and blue chips. If a chip is 7 15 Dec 2010, 08:11
A box contains red and blue balls only. If there are 8 balls 1 27 Nov 2010, 19:36
Display posts from previous: Sort by