Anahatha wrote:OG 11
Page:182
Problem # :217
(Sorry for not tying the entire question here)
The answer is given as 3/40000.
My doubt : why isn't the answer (3/40000) + (3/40000) since we can first choose a sibling from senior class then junior senior class OR junior class and then senior class.
Can someone please explain ? Thanks in advance..
I have this book:
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 will be a sibling pair.
(I'm too tired to type the options)
You find your answer by use the AND probability formula P(A)*P(B) since the question is asking, "What is the probability that we pick a junior that has a sibling that is a senior, AND then select that person's sibling?"
There are 60 sibling pairs, and there are 1,000 juniors, so the probability of selecting a Junior that is part of a sibling pair is 60/1,000
There are 800 seniors, and only one senior can be the selected junior's sibling, so the probability of making that selection is 1/800
P(A)*P(B) => 60/1,000*1/800 = 3/40,000
As for your question, it does not matter what order you make the random selection. Just as 2*3=6, 3*2=6. You'll always come up with the same number if you remember that the AND probability formula is applied to questions such as these.
WHY DO WE NEED TO MULTIPLY BY 2?
