Question

There are 1000 junior and 800 senior students in a class. And there are 60 sibling pairs where each pair has 1 junior and 1 senior.1 student is chosen from senior and 1 from junior randomly.What is the probability that the two selected students are from a sibling pair?

Answer 1

There are 1000 junior students of which 60 belong in a sibling pair. 

There are 800 senior students of which 60 belong in a sibling pair.

Thus, the chances of picking a student that is part of a sibling pair are 60/1000 and 60/800

Since you want to know the probability that BOTH the students are from a sibling pair, you'd have to multiply the probabilities together. So,

60/1000 x 60/800 = 9/2000

Hope this helps! <3

Answer 2

The options for this question are missing: Following are the options:

A) 7140/800000

B) 8140/800000

C) 7580/845000

D) 7540/800000

Answer: 7140/800000

Number of junior students: 1000

Number of Senior Students: 800

60 sibling pair = 2 x 60 = 120 students

The probability of the student being chosen from 800 and 1000 students is 1.

1 student chosen from senior and 1 student chosen from junior

n(s) = 800 x 1000 = 800000

n(E) = 120C2 = 7140

P(E) = n(E)/n(S) = 7140⁄800000