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

It is currently 19 Sep 2014, 02:18

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

Set S = If a number is selected from set S at random and

  Question banks Downloads My Bookmarks Reviews Important topics  
Author Message
TAGS:
CEO
CEO
User avatar
Joined: 21 Jan 2007
Posts: 2770
Location: New York City
Followers: 8

Kudos [?]: 232 [0], given: 4

GMAT Tests User
Set S = If a number is selected from set S at random and [#permalink] New post 24 Oct 2007, 11:56
Set S = [2,3,5,7]

If a number is selected from set S at random and then two numbers are subsequently selected (with replacement after each selection) what is the probability that the sum of these 3 numbers picked is odd?
VP
VP
User avatar
Joined: 08 Jun 2005
Posts: 1157
Followers: 6

Kudos [?]: 114 [0], given: 0

GMAT Tests User
 [#permalink] New post 24 Oct 2007, 12:04
o = odd
e = even

o+o+e or e+o+o or o+e+o or e+e+e

(3/4*3/4*1/4)*3 = 27/64

1/4*1/4*1/4 = 1/64

total

1 - 27/64+1/64 = 1 - 28/64 = 1- 7/16 = 9/16

:)
Senior Manager
Senior Manager
User avatar
Joined: 04 Jan 2006
Posts: 280
Followers: 1

Kudos [?]: 17 [0], given: 0

GMAT Tests User
Re: Probability [#permalink] New post 24 Oct 2007, 12:32
bmwhype2 wrote:
Set S = [2,3,5,7]

If a number is selected from set S at random and then two numbers are subsequently selected (with replacement after each selection) what is the probability that the sum of these 3 numbers picked is odd?


Yep, I did wrong. Edit my post :P

Here how I think the question is asking. (Correct me if I am wrong)
1. Pick any number from a set
2. Put the number back
3. Pick 2 numbers together from the set
4. Sum all 3 numbers from Step 1. and 3.

If the first pick is 2, which is even number, numbers picked from step 3 can only be 2 and another odd number
Possible ways = 1 x 1 x 3 = 3

If the first pick is odd number (3, 5, or 7), numbers picked from step 3 can only be two odds number.
Possible ways = 3 x (3C2) = 3 x 3 = 9

n(E) = 9 + 3 = 12

n(S) = 4 x (4C2) = 4 x 6 = 24

Prob = 12/24 = 1/2

Last edited by devilmirror on 24 Oct 2007, 13:20, edited 4 times in total.
VP
VP
User avatar
Joined: 08 Jun 2005
Posts: 1157
Followers: 6

Kudos [?]: 114 [0], given: 0

GMAT Tests User
 [#permalink] New post 24 Oct 2007, 12:48
another way:

1 - ((4C2*4C1 + 4C3)/4^3) = 1-(28/64) = 36/64 = 9/16

:)
VP
VP
User avatar
Joined: 08 Jun 2005
Posts: 1157
Followers: 6

Kudos [?]: 114 [0], given: 0

GMAT Tests User
 [#permalink] New post 24 Oct 2007, 23:36
what is the OA ?

:)
Senior Manager
Senior Manager
User avatar
Joined: 11 Sep 2005
Posts: 331
Followers: 1

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

GMAT Tests User
Re: Probability [#permalink] New post 25 Oct 2007, 01:58
bmwhype2 wrote:
Set S = [2,3,5,7]

If a number is selected from set S at random and then two numbers are subsequently selected (with replacement after each selection) what is the probability that the sum of these 3 numbers picked is odd?


2+3/2+5/2+7 => (3X3)/(4C1X4C1) => 9/16
CEO
CEO
User avatar
Joined: 21 Jan 2007
Posts: 2770
Location: New York City
Followers: 8

Kudos [?]: 232 [0], given: 4

GMAT Tests User
 [#permalink] New post 25 Oct 2007, 04:58
KillerSquirrel wrote:
what is the OA ?

:)


There is no OA. I made this question up to challenge myself. Image
It is a variant of one of the Challenge questions.

I haevnt solved it yet. I will post my explanation later although it looks as if KS got it right.
VP
VP
User avatar
Joined: 08 Jun 2005
Posts: 1157
Followers: 6

Kudos [?]: 114 [0], given: 0

GMAT Tests User
 [#permalink] New post 25 Oct 2007, 09:52
bmwhype2 wrote:
KillerSquirrel wrote:
what is the OA ?

:)


There is no OA. I made this question up to challenge myself. Image
It is a variant of one of the Challenge questions.

I haevnt solved it yet. I will post my explanation later although it looks as if KS got it right.


I like people who write their own questions for self improvement. This is the best way to better understand, but you should write some answer choices too.

:)
Current Student
User avatar
Joined: 08 Oct 2007
Posts: 169
Location: Berkeley, CA
Schools: Berkeley-Haas MBA
WE 1: Investment Management (fund of funds)
WE 2: Private Equity ($2bn generalist fund)
Followers: 1

Kudos [?]: 15 [0], given: 7

Re: Probability [#permalink] New post 25 Oct 2007, 10:24
bmwhype2 wrote:
Set S = [2,3,5,7]

If a number is selected from set S at random and then two numbers are subsequently selected (with replacement after each selection) what is the probability that the sum of these 3 numbers picked is odd?



I'm not sure if I understood the question correctly, because if I did, then I don't understand everyone's explanation.

The question said - with replacement after each selection

There were only 2 selections - first one was 1 number, second one was 2 numbers.

The only way one can get an odd number is if you sum 3 odd numbers...

so,

probability of sum being odd = P(1st pick is odd) * P(after 1st pick is put back, 2nd and 3rd picks are odd)

this equates to: 3/4 * (3/4 * 2/3) = 3/8

So the probability of picking 3 numbers is 3/8
CEO
CEO
User avatar
Joined: 21 Jan 2007
Posts: 2770
Location: New York City
Followers: 8

Kudos [?]: 232 [0], given: 4

GMAT Tests User
Re: Probability [#permalink] New post 25 Oct 2007, 10:34
stopper5 wrote:
bmwhype2 wrote:
Set S = [2,3,5,7]

If a number is selected from set S at random and then two numbers are subsequently selected (with replacement after each selection) what is the probability that the sum of these 3 numbers picked is odd?



I'm not sure if I understood the question correctly, because if I did, then I don't understand everyone's explanation.

The question said - with replacement after each selection

There were only 2 selections - first one was 1 number, second one was 2 numbers.

The only way one can get an odd number is if you sum 3 odd numbers...

so,

probability of sum being odd = P(1st pick is odd) * P(after 1st pick is put back, 2nd and 3rd picks are odd)

this equates to: 3/4 * (3/4 * 2/3) = 3/8

So the probability of picking 3 numbers is 3/8


sorry for the shoddy wording. it should be 3 selections with replacement after each one.
Manager
Manager
avatar
Joined: 18 Jun 2007
Posts: 55
Followers: 1

Kudos [?]: 1 [0], given: 0

 [#permalink] New post 25 Oct 2007, 17:26
odd = e + e + o
odd = o + o + o
That is the only way:

I added these two probabilities:
(1/4)(1/4)(3/4)+(3/4)(3/4)(3/4)
=3/64 + 27/64 = 30/64 = 15/32

Does that sound right?
VP
VP
User avatar
Joined: 08 Jun 2005
Posts: 1157
Followers: 6

Kudos [?]: 114 [0], given: 0

GMAT Tests User
 [#permalink] New post 25 Oct 2007, 23:10
ben928 wrote:
odd = e + e + o
odd = o + o + o
That is the only way:

I added these two probabilities:
(1/4)(1/4)(3/4)+(3/4)(3/4)(3/4)
=3/64 + 27/64 = 30/64 = 15/32

Does that sound right?


You forgot o+e+e and e+e+o are also odds:

1/4*1/4*3/4 = 3/64

3/4*1/4*1/4 = 3/64

1/4*3/4*1/4 = 3/64

3/4*3/4*3/4 = 27/64

total

9/64+27/64 = 36/64 = 9/16

:)
Senior Manager
Senior Manager
avatar
Joined: 04 Jun 2007
Posts: 374
Followers: 1

Kudos [?]: 13 [0], given: 0

GMAT Tests User
 [#permalink] New post 27 Oct 2007, 15:22
possibilities for odd are-

o+o+o= 3/4*3/4*3/4= 27/64
o+e+e= 3/4*1/4*1/4= 3/64
e+e+o= 3/64
e+o+e= 3/64

total= 9/64+27/64= 36/64= 9/16
CEO
CEO
User avatar
Joined: 21 Jan 2007
Posts: 2770
Location: New York City
Followers: 8

Kudos [?]: 232 [0], given: 4

GMAT Tests User
 [#permalink] New post 14 Dec 2007, 09:13
alright, finally got around to solving this questiion. i forgot i posted it.

here's the clearer version of the question:

Set S = [2,3,5,7]

If 3 numbers are selected (with replacement after each selection), what is the probability that the sum of these 3 numbers picked is odd?




KS is correct.
the OA is 36/64 or 9/16
Intern
Intern
avatar
Joined: 13 Jun 2007
Posts: 48
Followers: 1

Kudos [?]: 3 [0], given: 0

 [#permalink] New post 14 Dec 2007, 13:54
Awkward wording of the problem!!!

The way i initially understood the question, is that the 1st is not replace and the 2nd and 3rd selection are replaced.

So to have the probability of odd sum = 1 - probability of even
if 2 is selected first then 1/4*3/3*3/3=1/4
if odd is selected first then 3/4*1/3*1/3=1/12

1-1/3=2/3 probability of odd
  [#permalink] 14 Dec 2007, 13:54
    Similar topics Author Replies Last post
Similar
Topics:
2 Experts publish their posts in the topic One number, n, is selected at random from a set of 10 intege rajatr 3 05 May 2013, 21:20
Experts publish their posts in the topic A number is to be selected at random from the set above Stiv 2 01 May 2012, 01:30
One number, k, is selected at random from a set of 11 consec Economist 4 25 Oct 2009, 04:16
If x is to be selected at random from set T, what is the haas_mba07 6 06 Sep 2006, 19:37
A number is selected at random from the set of integers{ cloaked_vessel 4 25 Sep 2005, 08:20
Display posts from previous: Sort by

Set S = If a number is selected from set S at random and

  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®.