PS Question # 2 - Siblings

[sumit.sinha]

Answer Choices:
(A) 3/40,000
(B) 3/20,000
(C) 3/4,000
(D) 9/400
(E) 6/130

Source - VeritasPrep Practice Test





Answer Choices:
(A) 3/40,000
(B) 1/3,600
(C) 9/2,000
(D) 1/60
(E) 1/15

Source - GMATPrep Practice Test

OA - Soon

PS: Please click on the image to view the complete question.

[goyalsau]

A is the answer

[Rahul@gurome]

Basic principle of both the question is same. (Incidentally the answer is same too! )
I'll explain the first one.

Number of possible ways to select two students such that one of them is a junior student and another is a senior student = (Number of possible ways to select a junior student out of 1000)*(Number of possible ways to select a senior student out of 800) = 1000*800

Number of possible ways to select a sibling pair = (Number of possible ways to select one of them out of 60)*(Number of possible ways to select other one of them out of 60) = 60*1

(Number of possible ways to select other one of them out of 60 is 1 because as the first one is already selected other one is fixed for him/her.)

Therefore required probability = 60/(800*1000) = 3/40,000

The correct answer is A.

[sumit.sinha]

Basic principle of both the question is same. (Incidentally the answer is same too! )
I'll explain the first one.

Number of possible ways to select two students such that one of them is a junior student and another is a senior student = (Number of possible ways to select a junior student out of 1000)*(Number of possible ways to select a senior student out of 800) = 1000*800

Number of possible ways to select a sibling pair = (Number of possible ways to select one of them out of 60)*(Number of possible ways to select other one of them out of 60) = 60*1

(Number of possible ways to select other one of them out of 60 is 1 because as the first one is already selected other one is fixed for him/her.)

Therefore required probability = 60/(800*1000) = 3/40,000

The correct answer is A.

Thanks for the explanation Rahul.
But dont i need to take into consideration the way/sequence in which i select the siblings??
i.e in one sequence- i first select a sibling form Junior class and then select the matching one from senior class.
and in the 2nd sequence- i first select a sibling form senior class and then select the matching one from junior class