A bag contains 8 red marbles, 4 white marbles, and 5 blue marbles. You draw one marble at random. Find P(red and blue).

I think you mean P(red or blue), not P(red and blue), right? The probability of success in a random draw is equal to the number of objects that match the deifintion of success (in this case the number of marbles that are either red or blue) divided by the total population.

BTW, if indeed the question was phrased as P(red and blue) since you are drawing only one marble success is defined as drawing a marble that is both red and blue - so to answer the question you need to determine how many marbles that are bicolored like that.