Question

Suppose you choose a marble from a bag containing 3 red marbles, 5 white marbles, and 4 blue marbles. You return the first marble to the bag and then choose again. What is the probability that you will pick a red marble and then a blue marble?

Answer 1

`1/12`

Explanation:

Because you are returning the picks to the bag, the individual probability of selection is the same every time. For red it is
`3/12` : for Blue it is `4/12`.

The probability that you will pick one in sequence of the other is the multiple of the two, because it depends on FIRST the probability of the red selection, and then on the probability of a blue selection.
`3/12 xx 4/12 = 12/144 = 1/12`