A certain jar contains only b black marbles

Expert replies

rakeshd347: Master | Next Rank: 500 Posts; Posts: 391; Joined: Sat Mar 02, 2013 5:13 amFollowed by: 4 members; Thanked: 50 times

by rakeshd347 » Sat Oct 05, 2013 5:55 pm

A certain jar contains only b black marbles, w white marbles and r red marbles. If one marble is to be chosen at random from the jar, is the probability that the marble chosen will be red greater then the probability that the marble chosen will be white?

(1) r/(b+w) > w/(b+r)
(2) b-w > r

Like this post

Source: Beat The GMAT — Data Sufficiency | Sat Oct 05, 2013 5:55 pm

theCodeToGMAT: Legendary Member; Posts: 1556; Joined: Tue Aug 14, 2012 11:18 pmFollowed by: 34 members; Thanked: 448 times; GMAT Score:650

by theCodeToGMAT » Sat Oct 05, 2013 6:30 pm

To find: R>W
Statement 1:
r/(b+w) > w/(b+r)
Add "1" both sides
(r + b + w)/(b+w) > (r + b + w)/(b+r)
b + w < b + r
w<r
SUFFICIENT

Statement 2:
b-w>r
b+r>w
INSUFFICIENT

Answer [spoiler]{A}[/spoiler]

Last edited by theCodeToGMAT on Mon Oct 07, 2013 3:04 am, edited 1 time in total.

R A H U L

Like this post

GMAT/MBA Expert

[email protected]: Elite Legendary Member; Posts: 10392; Joined: Sun Jun 23, 2013 6:38 pmFollowed by: 511 members; Location: Palo Alto, CA; Thanked: 2867 times; GMAT Score:800

by [email protected] » Sun Oct 06, 2013 12:34 am

Hi rakeshd347,

For this DS question, you can use a combination of Number Properties and TESTing Values to find the correct answer.

We're told of 3 types of marbles (B, W and R). We're asked if the probability of pulling an R is > the probability of pulling a W. This is a Yes/No question.

Fact 1: R/(B+W) > W/(B+R)

You'll notice that the "B" is in the denominator of BOTH fractions, so we can focus on the R and W. You can TEST Values to find the pattern or use a Number Property.

If you TEST Values:
R = 3
W = 2
B = 1
The answer to the question is YES.

With a bit of effort, you'll find that R CANNOT EQUAL W (if it did, then the two fractions in Fact 1 would be equal).
Also, IF the W was bigger than the R, the first fraction WOULDN'T be greater than the second fraction.
These two Number Properties ELIMINATE the possibility that R = W and ELIMINATE the possibility that R < W.
Fact 1 is ALWAYS going to give you a YES answer.
Fact 1 is SUFFICIENT

Fact 2: B - W > R

Try TESTing Values
B = 3
W = 1
R = 1
The answer to the question is NO

B = 4
W = 1
R = 2
The answer to the question is YES
Fact 2 is INSUFFICIENT

Final Answer:A

GMAT assassins aren't born, they're made,
Rich

Contact Rich at [email protected]

Like this post

vipulgoyal: Master | Next Rank: 500 Posts; Posts: 468; Joined: Mon Jul 25, 2011 10:20 pmFollowed by: 4 members; Thanked: 29 times

by vipulgoyal » Mon Oct 07, 2013 2:58 am

Hi Rahul,

b + w > b + r
w>r
SUFFICIENT

i think here above you did some typo error
correction
b + w < b + r
w < r
SUFFICIENT

Like this post

theCodeToGMAT: Legendary Member; Posts: 1556; Joined: Tue Aug 14, 2012 11:18 pmFollowed by: 34 members; Thanked: 448 times; GMAT Score:650

by theCodeToGMAT » Mon Oct 07, 2013 3:03 am

vipulgoyal wrote:Hi Rahul,

b + w > b + r
w>r
SUFFICIENT

i think here above you did some typo error
correction
b + w < b + r
w < r
SUFFICIENT

Yes, typo... its tough to do maths on MS Notepad

.. issue fixed

R A H U L

Like this post

Mission2012: Master | Next Rank: 500 Posts; Posts: 184; Joined: Sun Aug 19, 2012 10:04 pmFollowed by: 2 members; Thanked: 10 times

by Mission2012 » Mon Oct 07, 2013 5:02 am

rakeshd347 wrote:A certain jar contains only b black marbles, w white marbles and r red marbles. If one marble is to be chosen at random from the jar, is the probability that the marble chosen will be red greater then the probability that the marble chosen will be white?

(1) r/(b+w) > w/(b+r)
(2) b-w > r

@ Rahul - Awesome solution

@ Rakesh - what is the source of this question

If you find my post useful -> please click on "Thanks"

Like this post

theCodeToGMAT: Legendary Member; Posts: 1556; Joined: Tue Aug 14, 2012 11:18 pmFollowed by: 34 members; Thanked: 448 times; GMAT Score:650

by theCodeToGMAT » Mon Oct 07, 2013 5:32 am

Mission2012 wrote:
rakeshd347 wrote:A certain jar contains only b black marbles, w white marbles and r red marbles. If one marble is to be chosen at random from the jar, is the probability that the marble chosen will be red greater then the probability that the marble chosen will be white?

(1) r/(b+w) > w/(b+r)
(2) b-w > r
@ Rahul - Awesome solution

@ Rakesh - what is the source of this question

Thanks

R A H U L

Like this post

GMAT/MBA Expert

lunarpower: GMAT Instructor; Posts: 3380; Joined: Mon Mar 03, 2008 1:20 amFollowed by: 1535 members; Thanked: 2256 times; GMAT Score:800

by lunarpower » Tue Oct 08, 2013 3:28 am

what is the source of this question

This problem has been in the GMAT PREP software for several years.

Ron has been teaching various standardized tests for 20 years.

--

Pueden hacerle preguntas a Ron en castellano
Potete chiedere domande a Ron in italiano
On peut poser des questions Ã Ron en franÃ§ais
Voit esittÃ¤Ã¤ kysymyksiÃ¤ Ron:lle myÃ¶s suomeksi

--

Quand on se sent bien dans un vÃªtement, tout peut arriver. Un bon vÃªtement, c'est un passeport pour le bonheur.

Yves Saint-Laurent

--

Learn more about ron

Like this post

GMATGuruNY: GMAT Instructor; Posts: 15539; Joined: Tue May 25, 2010 12:04 pmFollowed by: 1906 members; Location: New York, NY; Thanked: 13060 times; GMAT Score:790

by GMATGuruNY » Tue Oct 08, 2013 5:10 am

rakeshd347 wrote:A certain jar contains only b black marbles, w white marbles and r red marbles. If one marble is to be chosen at random from the jar, is the probability that the marble chosen will be red greater then the probability that the marble chosen will be white?

(1) r/(b+w) > w/(b+r)
(2) b-w > r

Very little math is needed here.
Just use common sense.

Question stem, rephrased: Is R>W?

Statement 1: r/(b+w) > w/(b+r)
Put into words:
The ratio of R to the OTHER marbles is greater than the ratio of W to the OTHER marbles.
Thus, R>W.
SUFFICIENT.

Statement 2: b-w > r
B > R+W.
No way to determine whether R>W.
INSUFFICIENT.

The correct answer is A.

Private tutor exclusively for the GMAT and GRE, with over 20 years of experience.
Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.

As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.

For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3

Like this post

Post new topic Post Reply

• Page 1 of 1

GMAT Course Reviews

560 reviews

1168 reviews

431 reviews

359 reviews

147 reviews

5/5

5 Star (560 Reviews)

"Target Test Prep is the closest to the official version of the GMAT exam, about 99% accuracy in terms of the quality and quantity of information. The course has excellently created singular sets of focused lessons and tests for every possible topic that one could come across in the official GMAT exam."

"The TTP course maximizes the efficiency of the time you spend studying. It will take time and effort but I could almost guarantee that if you complete the course exactly as it is laid out you will get an amazing score. They also have a very responsive team willing to help with any questions you might have."

"TTP has two things that I think no other test prep company offers: A teaching approach that reinforces understanding and an attitude that will give you the mental preparedness needed to succeed on the test. TTP gives you a deep understanding of the concept you need to know while teaching you how to think."

Write a Review