A box contains 20 cassettes, 4 of which are defective ...

 A box contains 20 cassettes, 4 of which are defective. If 2 cassettes are selected at random from the box, without replacing them, find the following:
 

a. Generate a tree diagram of the possible outcomes with their associated probabilities.
 

b. What is the probability that both are defective?
 

c. What is the probability that one is good and one is defective?
 

d. What is the probability that both are good?