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

 It is currently 07 Mar 2014, 11:55

### 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 certain junior class has 1,000 students and a certain

Author Message
TAGS:
Intern
Joined: 24 Feb 2012
Posts: 33
Followers: 0

Kudos [?]: 5 [0], given: 18

Re: A certain junior class has 1,000 students and a certain [#permalink]  24 Feb 2012, 13:10
60/1000 * 1/800
=3/40000

Alternate Technique:
\frac{Desired}{Total} = \frac{60}{1000 * 800}
= 3/40000

Solution to Bunuel's extension:
\frac{120}{10000} * \frac{2}{800}
=\frac{3}{10000}
VP
Status: Final Lap Up!!!
Affiliations: NYK Line
Joined: 21 Sep 2012
Posts: 1078
Location: India
GMAT 1: 410 Q35 V11
GMAT 2: 530 Q44 V20
GMAT 3: 630 Q45 V31
GPA: 3.84
WE: Engineering (Transportation)
Followers: 25

Kudos [?]: 208 [0], given: 65

Re: A certain junior class has 1,000 students and a certain [#permalink]  05 Oct 2012, 06:58
Can anyone clear my doubt i a m struggling
TOtal number of outcome = 1000*800
Sibbling with Juniors = 60
so 60 out of 1000

Sibling with Senior = 60
so 60 out of 800
we have to find probability of 2 sibbling 1 from each
60 C 1= Selection from Senior
60 C 1 = From Junior
total fav = 1000*800
Prob ={ 60 C 1 *60 C 1}/1000*800
=9/2000

pls tell me where i am wrong in my approach and what correction needed
Intern
Joined: 16 Jan 2013
Posts: 44
Concentration: Finance, Entrepreneurship
GMAT Date: 08-25-2013
Followers: 0

Kudos [?]: 10 [0], given: 8

Re: junior [#permalink]  12 Jul 2013, 22:12
eschn3am wrote:
$formdata=\frac{60*1}{1000*800}+=+\frac{60}{800000}=\frac{3}{40000}$

Let's say you're picking out of the Junior class first and the senior class second (although the order doesn't make any difference). There are 1000 juniors and 60 of them have a sibling in the senior class, so you have a $formdata=\frac{60}{1000}$ shot of choosing one of the siblings. Then you move onto the senior class. There are 800 seniors and only one sibling of the person you chose from the junior class. Thus, you have a $formdata=\frac{1}{800}$ chance of choosing the sibling.

Hi ,

Don't we need to consider the case where we pick the senior class first and then the junior class.

Plz clarify.

Math Expert
Joined: 02 Sep 2009
Posts: 16782
Followers: 2769

Kudos [?]: 17562 [0], given: 2183

Re: junior [#permalink]  12 Jul 2013, 23:16
Expert's post
Countdown wrote:
eschn3am wrote:
$formdata=\frac{60*1}{1000*800}+=+\frac{60}{800000}=\frac{3}{40000}$

Let's say you're picking out of the Junior class first and the senior class second (although the order doesn't make any difference). There are 1000 juniors and 60 of them have a sibling in the senior class, so you have a $formdata=\frac{60}{1000}$ shot of choosing one of the siblings. Then you move onto the senior class. There are 800 seniors and only one sibling of the person you chose from the junior class. Thus, you have a $formdata=\frac{1}{800}$ chance of choosing the sibling.

Hi ,

Don't we need to consider the case where we pick the senior class first and then the junior class.

Plz clarify.

These posts might help:
a-certain-junior-class-has-1-000-students-and-a-certain-58914.html#p778756
a-certain-junior-class-has-1-000-students-and-a-certain-58914.html#p812392
_________________
Intern
Joined: 07 Jan 2013
Posts: 43
Location: India
Concentration: Finance, Strategy
GMAT 1: 570 Q46 V23
GMAT 2: 710 Q49 V38
GPA: 2.9
WE: Information Technology (Computer Software)
Followers: 0

Kudos [?]: 7 [0], given: 23

Re: A certain junior class has 1,000 students and a certain [#permalink]  23 Jul 2013, 18:22
i just wanted to validate this answer the other way around i,e to find the no. of ways at least one sibling or no sibling is present in the chosen 2 members and then subtracting from 1 i.e

Req prob =1- (prob of one sibling chosen from either class) + no sibling chosen from either class
= 1-(prob of one sibling from junior) + (prob of one siblng from senior ) + (no sibling)
= 1-(\frac{60}{1000}*\frac{799}{800})+(\frac{60}{800}*\frac{999}{1000})+(\frac{740}{800}*\frac{940}{1000})

which is coming out to be -ve which obviously is wrong ,, i want to know as to what i am adding extra as a result the answer is -ve
_________________

Manager
Joined: 11 Jan 2011
Posts: 72
GMAT 1: 680 Q44 V39
GMAT 2: 710 Q48 V40
Followers: 0

Kudos [?]: 2 [0], given: 3

Re: junior [#permalink]  11 Nov 2013, 18:44
Bunuel wrote:
blog wrote:
A certain junior class has 1,000 students and a certain senior class has 800 students. Among these students, there are 60 siblings pairs , each consisting of 1 junior and 1 senior. If i student is to be selected at random from each class, what is the probability that the 2 students selected at will be a sibling pair?

A. 3/40,000
B. 1/3,600
C. 9/2,000
D. 1/60
E. 1/15

There are 60 siblings in junior class and 60 their pair siblings in the senior class. We want to determine probability of choosing one sibling from junior class and its pair from senior.

What is the probability of choosing ANY sibling from junior class? \frac{60}{1000} (as there are 60 of them).

What is the probability of choosing PAIR OF CHOSEN SIBLING in senior class? As in senior class there is only one pair of chosen sibling it would be \frac{1}{800} (as there is only one sibling pair of chosen one).

So the probability of that the 2 students selected will be a sibling pair is: \frac{60}{1000}*\frac{1}{800}=\frac{3}{40000}

This problem can be solved in another way:

In how many ways we can choose 1 person from 1000: C^1_{1000}=1000;
In how many ways we can choose 1 person from 800: C^1_{800}=800;
So total # of ways of choosing 1 from 1000 and 1 from 800 is C^1_{1000}*C^1_{800}=1000*800 --> this is total # of outcomes.

Let’s count favorable outcomes: 1 from 60 - C^1_{60}=60;
The pair of the one chosen: C^1_1=1
So total # of favorable outcomes is C^1_{60}*C^1_1=60

Probability=\frac{# \ of \ favorable \ outcomes}{Total \ # \ of \ outcomes}=\frac{60}{1000*800}=\frac{3}{40000}.

Let’s consider another example:
A certain junior class has 1000 students and a certain senior class has 800 students. Among these students, there are 60 siblings of four children, each consisting of 2 junior and 2 senior (I’m not sure whether it’s clear, I mean there are 60 brother and sister groups, total 60*4=240, two of each group is in the junior class and two in the senior). If 1 student is to be selected at random from each class, what is the probability that the 2 students selected will be a sibling pair?

The same way here:

What is the probability of choosing ANY sibling from junior class? 120/1000 (as there are 120 of them).
What is the probability of choosing PAIR OF CHOSEN SIBLING in senior class? As in senior class there is only two pair of chosen sibling it would be 2/800 (as there is only one sibling pair of chosen one).

So the probability of that the 2 students selected will be a sibling pair is: 120/1000*2/800=3/10000

Another way:
In how many ways we can choose 1 person from 1000=1C1000=1000
In how many ways we can choose 1 person from 800=1C800=800
So total # of ways of choosing 1 from 1000 and 1 from 800=1C1000*1C800=1000*800 --> this is our total # of outcomes.

Favorable outcomes:
1 from 120=120C1=120
The pair of the one chosen=1C2=2
So total favorable outcomes=120C1*1C2=240

Probability=Favorable outcomes/Total # of outcomes=240/(1000*800)=3/10000

Also discussed at: probability-85523.html?hilit=certain%20junior%20class#p641153

Hi Bunuel,

In your second example, how is it possible to have the equation bolded above: 1c2?

Thanks,
Rich
Intern
Joined: 05 Dec 2013
Posts: 2
Followers: 0

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

Re: junior [#permalink]  12 Dec 2013, 04:58
jeeteshsingh wrote:
blog wrote:
A certain junior class has 1,000 students and a certain senior class has 800 students. Among these students, there are 60 siblings pairs , each consisting of 1 junior and 1 senior. If i student is to be selected at random from each class, what is the probability that the 2 students selected at will be a sibling pair?

A. 3/40,000
B. 1/3,600
C. 9/2,000
D. 1/60
E. 1/15

No of ways of choosing 1 sibling pair out of 60 pairs = 60c1
No of ways of choosing 1 student from each class = 1000c1 x 800c1

Therefore probability of having 2 students choosen as a sibling pair = 60c1 / (1000c1 x 800c1) = 60 / (1000 x 800) = 3 / 40000 = A

This was What I thought when I solved this question
Intern
Joined: 21 Apr 2013
Posts: 8
Schools: Kelley '16, AGSM '15
Followers: 0

Kudos [?]: 1 [0], given: 24

Re: A certain junior class has 1,000 students and a certain [#permalink]  31 Jan 2014, 13:18
Archit143 wrote:
Can anyone clear my doubt i a m struggling
TOtal number of outcome = 1000*800
Sibbling with Juniors = 60
so 60 out of 1000

Sibling with Senior = 60
so 60 out of 800
we have to find probability of 2 sibbling 1 from each
60 C 1= Selection from Senior
60 C 1 = From Junior
total fav = 1000*800
Prob ={ 60 C 1 *60 C 1}/1000*800
=9/2000

pls tell me where i am wrong in my approach and what correction needed

For the selection from Junior we would have only 1 and not 60 C 1. This is because since we have chosen someone from the senior class and now the only choice we have is the sibling of the senior student we chose.
Re: A certain junior class has 1,000 students and a certain   [#permalink] 31 Jan 2014, 13:18
Similar topics Replies Last post
Similar
Topics:
A certain junior class has 1000 students and a certain 4 10 Sep 2004, 19:15
A certain junior class has 1000 students and a certain 3 19 Feb 2006, 12:58
A certain junior class has 1000 students and a certain 3 14 Apr 2006, 18:36
A certain junior class has 1000 students and a certain 1 07 May 2006, 17:04
A certain junior class has 1000 students and a certain 1 20 May 2006, 04:45
Display posts from previous: Sort by