IF there are 60pairs, it means there are 60students in the senior class who have a sibling in the junior class.

In order to pick 1 pair, select 1 student fm the senior class who has a sibling
This can be done in 60/800 ways
This student has only 1 sibling in the junior class and can be picked in 1/1000ways

So total number of ways = (60/800) * (1/1000)

Hope this helps

The probability of an event to occur is defined as , p(E) = (number of required outcome)/(number of total outcome)
probability of two events, A and B occurring at the same time is define as p(A) and p(B)= p(A) * p(B)

now, the number of junior students, n(j) = 1000
and number of senior students, n(s) = 800
There are sixty siblings pair, each from both classes.
i.e 60 students from the junior class have one sibling each in the same class.
let p(j) be the probability of a junior student having a sibling in the senior class.

p(j) = (no. of junior students that have a sibling) / (total number of junior students)
= 60/1000 = 3/50
Also, let P(s) be the probability of a senior class student having a sibling in the junior class

therefore, p(S)= 60/800 = 3/40
Now, the probability of one junior student and one senior student being sibling
= p(J) and p(S)
= p(J) * p(S)
= (3/50) * (3/40)
= 9/2000

Hence, option C is very correct :mrgreen: