Hello,

Consider the following two questions:

(a)
In a bag of marbles, there are 3 red, 2 white, and 5 blue. If Bob takes 2 marbles out of the bag, what is the probability that he will have one white and one blue marble?

(b)
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?

In question (a), you calculate the probability of picking a white marble first and then the blue marble and add it to the probability of picking a blue marble first, then picking the white marble. So you add two probabilities.

In question (b), you calculate the probability of picking a junior sibling pair (60/1000) and multiply by the probability of picking a senior sibling pair (1/800). Why don't you add the probability of picking the senior first, then the junior, as such in question (a)??? What's the fundamental difference between the two questions?

Thanks in advance!