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

 It is currently 14 Dec 2018, 00:46

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 December
PrevNext
SuMoTuWeThFrSa
2526272829301
2345678
9101112131415
16171819202122
23242526272829
303112345
Open Detailed Calendar
• GMATbuster's Weekly GMAT Quant Quiz, Tomorrow, Saturday at 9 AM PST

December 14, 2018

December 14, 2018

09:00 AM PST

10:00 AM PST

10 Questions will be posted on the forum and we will post a reply in this Topic with a link to each question. There are prizes for the winners.
• Typical Day of a UCLA MBA Student - Recording of Webinar with UCLA Adcom and Student

December 14, 2018

December 14, 2018

10:00 PM PST

11:00 PM PST

Carolyn and Brett - nicely explained what is the typical day of a UCLA student. I am posting below recording of the webinar for those who could't attend this session.

A certain junior class has 600 students and a certain senior class has

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

Hide Tags

Math Expert
Joined: 02 Sep 2009
Posts: 51204
A certain junior class has 600 students and a certain senior class has  [#permalink]

Show Tags

03 Jul 2018, 09:55
00:00

Difficulty:

95% (hard)

Question Stats:

40% (02:40) correct 60% (02:20) wrong based on 61 sessions

HideShow timer Statistics

A certain junior class has 600 students and a certain senior class has 800 students. Among these students, there are N sibling pairs, consisting of a Junior and a Senior. What is the value of N?

(1) If a student is selected from each class at random, the probability that the students do not form a sibling pair is 4799/4800.

(2) If a student is selected from each class at random, the probability that exactly ONE of these students is a member of a sibling pair is 1/4.

_________________
Math Expert
Joined: 02 Aug 2009
Posts: 7106
A certain junior class has 600 students and a certain senior class has  [#permalink]

Show Tags

03 Jul 2018, 12:08
2
A certain junior class has 600 students and a certain senior class has 800 students. Among these students, there are N sibling pairs, consisting of a Junior and a Senior. What is the value of N?

(1) If a student is selected from each class at random, the probability that the students do not form a sibling pair is 4799/4800.
$$\frac{4799}{4800}=\frac{479900}{480000}$$
So prob they form =$$\frac{100}{600*800}=\frac{N}{800}*\frac{1}{600}$$...
So N=100
Suff

(2) If a student is selected from each class at random, the probability that exactly ONE of these students is a member of a sibling pair is 1/4.
$$\frac{N}{600}* \frac{(800-N)}{800}+\frac{N}{800}*\frac{(600-N)}{600}=\frac{1}{4}$$......
How?? - since we are looking exactly one and there are N pairs, we choose any of the N in class of 600, so we will have to choose students who are not part of this sibling pairs so 800-N out of other class of 800. But there can be other way where we choose any of N from 800 and all but this N from 600 so 600-N
$$N(800-N)+N(600-N)=\frac{(600*800)}{4}=120000$$..
$$N(800-N+600-N)=120000........N^2-700N+60000=0$$
$$(N-100)(N-600)=0$$
N can be 100 or 600
Insufficient

A

rajudantuluri, Added the reasoning. Hope it helps
_________________

1) Absolute modulus : http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372
2)Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html
3) effects of arithmetic operations : https://gmatclub.com/forum/effects-of-arithmetic-operations-on-fractions-269413.html

GMAT online Tutor

Intern
Joined: 15 Aug 2012
Posts: 42
Schools: AGSM '19
A certain junior class has 600 students and a certain senior class has  [#permalink]

Show Tags

04 Jul 2018, 13:09
chetan2u wrote:
A certain junior class has 600 students and a certain senior class has 800 students. Among these students, there are N sibling pairs, consisting of a Junior and a Senior. What is the value of N?

(1) If a student is selected from each class at random, the probability that the students do not form a sibling pair is 4799/4800.
$$\frac{4799}{4800}=\frac{479900}{480000}$$
So prob they form =$$\frac{100}{600*800}=\frac{N}{800}*\frac{1}{600}$$...
So N=100
Suff

(2) If a student is selected from each class at random, the probability that exactly ONE of these students is a member of a sibling pair is 1/4
N/600 * (800-N)/800+ N/800*(600-N)/600=1/4......
N(800-N)+N(600-N)=(600*800)/4=120000..
N(800-N+600-N)=120000........N^2-700N+60000=0
(N-100)(N-600)=0
N can be 100 or 600
Insufficient

A

Hi chetan2u,
Could you please elaborate the reasoning behind the equation you set up in statement 2. I fail to understand why we do 800-N/800 and 600-N/600.
Math Expert
Joined: 02 Aug 2009
Posts: 7106
Re: A certain junior class has 600 students and a certain senior class has  [#permalink]

Show Tags

04 Jul 2018, 22:56
added the reasoning in original post.
Hope it helps
_________________

1) Absolute modulus : http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372
2)Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html
3) effects of arithmetic operations : https://gmatclub.com/forum/effects-of-arithmetic-operations-on-fractions-269413.html

GMAT online Tutor

Intern
Joined: 15 Aug 2012
Posts: 42
Schools: AGSM '19
Re: A certain junior class has 600 students and a certain senior class has  [#permalink]

Show Tags

05 Jul 2018, 07:46
chetan2u wrote:
added the reasoning in original post.
Hope it helps

Thank you chetan2u, that makes it very clear now
Re: A certain junior class has 600 students and a certain senior class has &nbs [#permalink] 05 Jul 2018, 07:46
Display posts from previous: Sort by

A certain junior class has 600 students and a certain senior class has

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

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