Three light bulbs are chosen at random from $15$ bulbs, of which five are defective. What is the probability that: 

1. None of the three are defective.

   Hint (a):

   Use combinations to solve this problem. For a review of combinations, see the Math Notes in Lesson 10.1.1.

   More Help (a):

   $\text{Remember that probability =} \frac{\text{# of desirable outcomes}}{\text{# of possible outcomes}}$.

   Answer (a):

   

2. Exactly two are defective.

   Hint (b):

   Use the same method as part (a).