A teacher has a bag of marbles. There are 6 red, 6 blue, 8 green, 7 purple, and 8
 yellow marbles in the bag. As the students enter the classroom, they draw a marble and keep it. If the first student in the room draws a yellow, and the second draws 
 a purple, what is the probability that the third student will draw a yellow also?
 33%
 21%
 20%
 25%