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

[BTGmoderatorDC]

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

[GMATGuruNY]

A certain junior class has 1000 students and a certain senior class has 800 students. among these students there are 60 sibling 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 2 students selected will be sibling pair
1) 3/40,000
2)1/3,600
3)9/2,000
4)1/60
5)1/15

P(sibling pair) = (total number of sibling pairs)/(total number of possible pairs).

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.

[Brent@GMATPrepNow]

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(selecting a sibling pair) = P(select a junior with a sibling AND select the senior who is that junior's sibling)
= 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

[Scott@TargetTestPrep]

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

In the junior class, the probability of selecting any one sibling from the 60 sibling pairs is 60/1000. Once that person is selected, the probability of selecting his or her sibling from the senior class is 1/800; thus, the probability of a selecting a sibling pair is:

60/1000 x 1/800 = 3/50 x 1/800 = 3/40000

Alternatively, in the senior class, the probability of selecting any one sibling from the 60 sibling pairs is 60/800. Once that person is selected, the probability of selecting his or her sibling from the junior class is 1/1000; thus, the probability of a selecting a sibling pair is:

60/800 x 1/1000 = 3/40 x 1/1000 = 3/40000

Answer: A