GMAT Question of the Day - Daily to your Mailbox; hard ones only

It is currently 15 Dec 2019, 16:21

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.

Close

Request Expert Reply

Confirm Cancel

A box contains 10 red balls and 30 blue balls. Balls will be pulled

  new topic post reply Question banks Downloads My Bookmarks Reviews Important topics  
Author Message
TAGS:

Hide Tags

Find Similar Topics 
Intern
Intern
avatar
B
Joined: 25 Oct 2016
Posts: 4
A box contains 10 red balls and 30 blue balls. Balls will be pulled  [#permalink]

Show Tags

New post Updated on: 07 Oct 2018, 05:40
2
00:00
A
B
C
D
E

Difficulty:

  35% (medium)

Question Stats:

67% (01:42) correct 33% (02:12) wrong based on 53 sessions

HideShow timer Statistics

A box contains 10 red balls and 30 blue balls. Balls will be pulled out of the box and then put back. It will stop when 2 blue balls have been pulled out of the box. What is the probability of getting at least 1 red ball before stopping?

A. 1/2
B. 2/3
C. 9/16
D. 3/4
E. 7/16

Originally posted by trankimphuong on 07 Oct 2018, 02:11.
Last edited by chetan2u on 07 Oct 2018, 05:40, edited 1 time in total.
Corrected the choices
Math Expert
avatar
V
Joined: 02 Aug 2009
Posts: 8326
A box contains 10 red balls and 30 blue balls. Balls will be pulled  [#permalink]

Show Tags

New post 07 Oct 2018, 05:39
1
trankimphuong wrote:
A box contains 10 red balls and 30 blue balls. Balls will be pulled out of the box and then put back. It will stop when 2 blue balls have been pulled out of the box. What is the probability of getting at least 1 red ball before stopping?

A. 1/2
B. 2/3
C. 9/6
D. 3/4
E. 7/6


Choices are clearly flawed. You can't have probability >1. Both C(9/6) and E(7/6) are >1..
So it should be 16 in denominator..

Easiest way is to find prob of both being BLUE, meaning NO red .
Probability of B one after another = \(\frac{30}{10+30}*\frac{30}{10+30}=\frac{3*3}{4*4}=\frac{9}{16}\)
This is the only way that Red is not picked up before the game stops. Otherwise the ways can be
1) B,R,R,B
2)R,R,B,B
3) R,R,R,R,R,B,R,B
And so on

So probability that atleast one red is picked = 1-(probability that NO red is picked)=1-9/16=7/16

E
_________________
Senior Manager
Senior Manager
User avatar
V
Joined: 22 Feb 2018
Posts: 413
Re: A box contains 10 red balls and 30 blue balls. Balls will be pulled  [#permalink]

Show Tags

New post 07 Oct 2018, 06:51
1
trankimphuong wrote:
A box contains 10 red balls and 30 blue balls. Balls will be pulled out of the box and then put back. It will stop when 2 blue balls have been pulled out of the box. What is the probability of getting at least 1 red ball before stopping?

A. 1/2
B. 2/3
C. 9/16
D. 3/4
E. 7/16


OA:E

Probability of getting at least 1 red ball before stopping \(= 1- \frac{30}{40}*\frac{30}{40}=1-\frac{9}{16}=\frac{7}{16}\)
GMAT Club Bot
Re: A box contains 10 red balls and 30 blue balls. Balls will be pulled   [#permalink] 07 Oct 2018, 06:51
Display posts from previous: Sort by

A box contains 10 red balls and 30 blue balls. Balls will be pulled

  new topic post reply Question banks Downloads My Bookmarks Reviews Important topics  





Powered by phpBB © phpBB Group | Emoji artwork provided by EmojiOne