A professor buys postage stamps and puts them in an envelope. Exactly 30 of the stamps are for letters, and exactly 10 of the stamps are for postcards. The professor's wife grabs two stamps. All outcomes are equally likely.
a. What is the probability that both stamps she selects are for letters?
b. What is the probability that both stamps she selects are for postcards?
c. What is the probability that she gets one of each type?