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.

It appears that you are browsing the GMAT Club forum unregistered!

Signing up is free, quick, and confidential.
Join other 500,000 members and get the full benefits of GMAT Club

Registration gives you:

Tests

Take 11 tests and quizzes from GMAT Club and leading GMAT prep companies such as Manhattan GMAT,
Knewton, and others. All are free for GMAT Club members.

Applicant Stats

View detailed applicant stats such as GPA, GMAT score, work experience, location, application
status, and more

Books/Downloads

Download thousands of study notes,
question collections, GMAT Club’s
Grammar and Math books.
All are free!

Thank you for using the timer!
We noticed you are actually not timing your practice. Click the START button first next time you use the timer.
There are many benefits to timing your practice, including:

There were initially no black marbles in a jar. Subsequently [#permalink]

Show Tags

05 Feb 2008, 23:29

8

This post was BOOKMARKED

00:00

A

B

C

D

E

Difficulty:

45% (medium)

Question Stats:

61% (02:05) correct
39% (01:00) wrong based on 302 sessions

HideShow timer Statistics

There were initially no black marbles in a jar. Subsequently, new marbles were added to the jar. If marbles are drawn at random and selected marbles are not returned to the jar, what is the probability of selecting 2 black marbles in a row?

(1) After the new marbles are added, 50% of all marbles are black. (2) Among the 10 added marbles, 8 are black.

There were initially no black marbles in a jar. Subsequently, new marbles were added to the jar. If marbles are drawn at random and the selected marbles are not returned to the jar, what is the probability of selecting 2 black marbles in a row?

1.after the new marbles are added, 50% of all marbles are black 2. among the 10 aded marbles, 8 are black

(C)

1) Could have 3 black and 3 white or 8 black and 8 white. Probability differs for both cases. Insufficient 2) Need to know how many white are there in the jar. This doesn't provide that information.

1) & 2) combined There are 8 black and 8 white in the jar. Now it should be easy! Sufficient
_________________

There were initially no black marbles in a jar. Subsequently, new marbles were added to the jar. If marbles are drawn at random and the selected marbles are not returned to the jar, what is the probability of selecting 2 black marbles in a row?

1.after the new marbles are added, 50% of all marbles are black 2. among the 10 aded marbles, 8 are black

1: INsuff. we dont know how many marbles were added. 2: Insuff. All we knwo is that 10 marbles were added to an amount of marbles. 8 of these new marbles are black.

Together, we know there must be a total of 16marbles. b/c 8 marbles = 1/2 of the entire jar.

There were initially no black marbles in a jar. Subsequently, new marbles were added to the jar. If marbles are drawn at random and the selected marbles are not returned to the jar, what is the probability of selecting 2 black marbles in a row?

1.after the new marbles are added, 50% of all marbles are black 2. among the 10 aded marbles, 8 are black

Kindly correct me where am I wrong ?

For me it should be D

1. No. of ways of selecting 1st & 2nd black marble : (3 black & 3 other) 3/6 * 2/5 ----- (45 black & 45 other) 45/90 * 44/89 ----- (1755 black & 1755 other) 1755/3510 * 1754/3509 ----

In all cases (P) = 0.5 * 0.4(approx. to 1st place decimal) ... suff.

2. No. of ways of selecting 1st & 2nd black marble : 8/10 * 7/9 = 28/45 ----- its very clearly specified that total marbles are 10 & 8 out of them are black ... suff.

1. No. of ways of selecting 1st & 2nd black marble : (3 black & 3 other) 3/6 * 2/5 ----- (45 black & 45 other) 45/90 * 44/89 ----- (1755 black & 1755 other) 1755/3510 * 1754/3509 ----

In all cases (P) = 0.5 * 0.4(approx. to 1st place decimal) ... suff.

2. No. of ways of selecting 1st & 2nd black marble : 8/10 * 7/9 = 28/45 ----- its very clearly specified that total marbles are 10 & 8 out of them are black ... suff.

In statement 1, your calculation in getting 0.4 is incorrect. Infact in your example 1754/3509 would be very very close to 0.5 itself. Basically this value could range from 0 to 0.5. In statement 2, total is not 10. 10 is only number of total marbles added. You still dont know how many were already present.

As shown by others earlier, choice C is the correct one.
_________________

In statement 1, your calculation in getting 0.4 is incorrect. Infact in your example 1754/3509 would be very very close to 0.5 itself. Basically this value could range from 0 to 0.5. In statement 2, total is not 10. 10 is only number of total marbles added. You still dont know how many were already present.

As shown by others earlier, choice C is the correct one.

Thanks for your reply Vips , I concede that B and A( Especially when total balls have lower values 2,4 --- the values do diverge beyond 0.4) are not sufficient..... but would you hold A not sufficient even if after the new balls are added , the total balls becomes 52 or greater ???

In statement 1, your calculation in getting 0.4 is incorrect. Infact in your example 1754/3509 would be very very close to 0.5 itself. Basically this value could range from 0 to 0.5. In statement 2, total is not 10. 10 is only number of total marbles added. You still dont know how many were already present.

As shown by others earlier, choice C is the correct one.

Thanks for your reply Vips , I concede that B and A( Especially when total balls have lower values 2,4 --- the values do diverge beyond 0.4) are not sufficient..... but would you hold A not sufficient even if after the new balls are added , the total balls becomes 52 or greater ???

Thats right. Even if number of balls is 500 or 600, the probability of such event would be different (though very close to each other). As long as you are getting multiple values instead of one unique solution (for the question asked - it could be a range asked in question) from a statement, just mark the option as insufficient.
_________________

There were initially no black marbles in a jar. Subsequently, new marbles were added to the jar. If marbles are drawn at random and the selected marbles are not returned to the jar, what is the probability of selecting 2 black marbles in a row?

1.after the new marbles are added, 50% of all marbles are black 2. among the 10 aded marbles, 8 are black

Kindly correct me where am I wrong ?

For me it should be D

1. No. of ways of selecting 1st & 2nd black marble : (3 black & 3 other) 3/6 * 2/5 ----- (45 black & 45 other) 45/90 * 44/89 ----- (1755 black & 1755 other) 1755/3510 * 1754/3509 ----

In all cases (P) = 0.5 * 0.4(approx. to 1st place decimal) ... suff.

2. No. of ways of selecting 1st & 2nd black marble : 8/10 * 7/9 = 28/45 ----- its very clearly specified that total marbles are 10 & 8 out of them are black ... suff.

You could consider easier cases for (1).

If after the new marbles are added, there is 1 black marble and 1 non-black marble in the jar (so if initially there was only 1 non-black marble and 1 black marble was added), then the probability of selecting 2 black marbles in a row will be 0 (since we have only 1 black marble).

But if after the new marbles are added, there are 2 black and 2 non-black marbles in the jar, then the probability of selecting 2 black marbles in a row will obviously be more than 0.

Re: There were initially no black marbles in a jar. Subsequently [#permalink]

Show Tags

31 Dec 2012, 03:02

1

This post received KUDOS

P ( 1 & 2 Black ) = P ( first black ) * P ( second black ) = ?

model of jar > Black + Others = Total P ( first b ) = b/t after selecting one marble and throwing it away, the model will change and total num of marbles will be reduced by one, so ... P (second black) = (b-1)/(t-1)

stmt 1. B = 50% * T

P( first black ) = b/t = 0.5 but we dont know t and b to find P ( sec b ) insuf.

Re: There were initially no black marbles in a jar. Subsequently [#permalink]

Show Tags

29 Oct 2015, 02:45

Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.
_________________

There were initially no black marbles in a jar. Subsequently [#permalink]

Show Tags

04 Aug 2016, 04:18

marcodonzelli wrote:

There were initially no black marbles in a jar. Subsequently, new marbles were added to the jar. If marbles are drawn at random and selected marbles are not returned to the jar, what is the probability of selecting 2 black marbles in a row?

(1) After the new marbles are added, 50% of all marbles are black. (2) Among the 10 added marbles, 8 are black.

(1) After the new marbles are added, 50% of all marbles are black. If only one draw was asked , then this info was sufficient, but here we have to draw two marbles one after the another without replacement First draw :- half marbles are black so probablity of drawing black marble is half marble (50% probablity) Second Draw :- Now we since we don't know exactly how many marbles are left in the jar we cannot find out the probablity of second draw. INSUFFICIENT (2) Among the 10 added marbles, 8 are black Since we do not know how many marbles are already present in the jar we do not know how adding 8 black and 2 other colour marbles will change the number of total marbles INSUFFICIENT

MERGE:- Since there were no marbles earlier therefore adding 8 marbles will result in a total of 8 marbles in the jar and these 8 marbles are half of the total marbles. therefore the other marbles are 8 in number. Now we know Black marbles = 8 Other marbles = 8 Total marbles = 16 Probability of picking two black marbles without replacement =\(\frac{8}{16}*\frac{7}{15}\)

SUFFICIENT

ANSWER IS C
_________________

Posting an answer without an explanation is "GOD COMPLEX". The world doesn't need any more gods. Please explain you answers properly. FINAL GOODBYE :- 17th SEPTEMBER 2016.

gmatclubot

There were initially no black marbles in a jar. Subsequently
[#permalink]
04 Aug 2016, 04:18

Its been long time coming. I have always been passionate about poetry. It’s my way of expressing my feelings and emotions. And i feel a person can convey...

Written by Scottish historian Niall Ferguson , the book is subtitled “A Financial History of the World”. There is also a long documentary of the same name that the...

Post-MBA I became very intrigued by how senior leaders navigated their career progression. It was also at this time that I realized I learned nothing about this during my...