A certain junior class has 1,000 students and a certain senior class has 800 students. Among these students, there are 6

[BTGModeratorVI]

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

Answer: A
Source: official guide

[deloitte247]

Junior class = 1000 students
Senior class = 800 students
60 siblings pairs each consist 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 will be a sibling pair?
Probability of selecting any sibling from Junior class
$$=\frac{no.\ of\ possible\ siblings\ pairs}{no.\ of\ possible\ juniors}=\frac{60}{1000}$$
In the senior class, there is just 1 pair of chosen siblings, so, the probability of choosing from senior class = 1/800
Probability that the 2 students selected will be a chosen pair
$$=\frac{60}{1000}\cdot\frac{1}{800}=\frac{3}{40,000}$$

Answer = option A