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 1 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
OA A
Source: GMAT Prep
A certain junior class has 1,000 students and a certain
This topic has expert replies
-
- Moderator
- Posts: 7187
- Joined: Thu Sep 07, 2017 4:43 pm
- Followed by:23 members
Timer
00:00
Your Answer
A
B
C
D
E
Global Stats
- GMATGuruNY
- GMAT Instructor
- Posts: 15539
- Joined: Tue May 25, 2010 12:04 pm
- Location: New York, NY
- Thanked: 13060 times
- Followed by:1906 members
- GMAT Score:790
P(sibling pair) = (total number of sibling pairs)/(total number of possible pairs).BTGmoderatorDC 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 1 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
Total number of possible pairs:
There are 1000 juniors and 800 seniors.
Total number of ways to combine 1 junior with 1 senior = 1000*800 = 800,000.
Total number of sibling pairs = 60.
Thus:
P(sibling pair) = 60/800,000 = 3/40,000.
The correct answer is A.
Alternate approach:
Junior class:
P(picking a member of a sibling pair) = 60/1000. (Of the 1000 juniors, 60 belong to a sibling pair.)
Senior class:
P(picking the selected junior's sibling) = 1/800. (Of the 800 seniors, 1 is the selected junior's sibling).
Since we want both events to happen, we MULTIPLY the probabilities:
(60/1000) * (1/800) = 60/800000 = 3/40000.
Private tutor exclusively for the GMAT and GRE, with over 20 years of experience.
Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.
As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.
For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3
Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.
As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.
For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3
GMAT/MBA Expert
- Brent@GMATPrepNow
- GMAT Instructor
- Posts: 16207
- Joined: Mon Dec 08, 2008 6:26 pm
- Location: Vancouver, BC
- Thanked: 5254 times
- Followed by:1268 members
- GMAT Score:770
P(selecting a sibling pair) = P(select a junior with a sibling AND select the senior who is that junior's sibling)BTGmoderatorDC 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 1 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
OA A
Source: GMAT Prep
= P(select a junior with a sibling) x P(select the senior who is that junior's sibling[/u])
= 60/1000 x 1/800
= 60/800,000
= 3/40,000
= A
Note: P(select a junior with a sibling) = 60/1000, because 60 of the 1000 juniors have a sibling who is a senior.
P(select a senior who is that junior's sibling) = 1/800, because there are 800 senior's and only 1 of them is the sibling of the selected junior.
Cheers,
Brent
We can try as follows,
Probability to get a sibling member on junior class = P1 = 60/1.000
Probability to get a sibling member on senior class = P2 = 60/800
Probability to get a sibling pair on all sibling pairs = P3 = 1/60
Probability that the 2 students selected at will be a sibling pair = P4
P4 = P1 * P2 * P3 = (60/1.000)*(60/800)*(1/60) = 3/40.000.
Hence, A is the correct answer. Regards!
Probability to get a sibling member on junior class = P1 = 60/1.000
Probability to get a sibling member on senior class = P2 = 60/800
Probability to get a sibling pair on all sibling pairs = P3 = 1/60
Probability that the 2 students selected at will be a sibling pair = P4
P4 = P1 * P2 * P3 = (60/1.000)*(60/800)*(1/60) = 3/40.000.
Hence, A is the correct answer. Regards!
GMAT/MBA Expert
- Scott@TargetTestPrep
- GMAT Instructor
- Posts: 7223
- Joined: Sat Apr 25, 2015 10:56 am
- Location: Los Angeles, CA
- Thanked: 43 times
- Followed by:29 members
BTGmoderatorDC 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 1 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
OA A
Source: GMAT Prep
Scott Woodbury-Stewart
Founder and CEO
[email protected]
See why Target Test Prep is rated 5 out of 5 stars on BEAT the GMAT. Read our reviews