The number of marbles of different colors stored in a hat is listed below:
 4 red marbles
 10 green marbles
 7 blue marbles
 Without looking in the hat, Dan takes out a marble at random. He replaces the marble and then takes out another marble from the hat. What is the probability that Dan takes out a blue marble in both draws?
 2/21
 1/9
 1/2
 2/3