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