A box in a supply room contains 15 compact fluorescent lightbulbs, of which 5 are rated 13-watt, 6 are rated 18-watt, and 4 are rated 23-watt. Suppose that three of these bulbs are randomly selected.
a. What is the probability that exactly two of the selected bulbs are rated 23-watt?
b. What is the probability that all three of the bulbs have the same rating?
c. What is the probability that one bulb of each type is selected?
d. If bulbs are selected one by one until a 23-watt bulb is obtained, what is the probability that it is necessary to examine at least 6 bulbs?