LulaBrazilia wrote:A certain junior class has 1,000 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 the 2 students selected will be a sibling pair?
A) 3/40,000
B) 1/3,600
C) 9/2,000
D) 1/60
E) 1/16
One option is to apply probability rules.
So, for two siblings to be selected, 2 things must happen: we must select a junior who has a senior sibling AND the senior selected must be the sibling of the selected junior.
We get P(
junior with sibling AND
selected senior is sibling to selected junior)= P(
junior with sibling) x P(
selected senior is sibling to selected junior)
P(junior with sibling): there are 1000 juniors and 60 of them have senior siblings. So, P(junior with sibling)=
60/1000
P(selected senior is sibling to selected junior): Once the junior has been selected, there is only 1 senior (out of 800 seniors) who is the sibling to the selected junior. So, P(selected senior is sibling to selected junior)=
1/800
So, the probability is
(60/1000)x
(1/800) = 3/40,000
The answer is
A
Cheers,
Brent
