a. A fair die is rolled 7 times. An "ace" means to roll a "6." Find the probability of getting three aces, of getting four aces, of getting five aces, of getting six aces, and of getting three aces or more.
b. A certain insurance salesman sees an average of eight customers in a week. Each time he speaks to a customer, he has a 20 % chance of making a deal. What is the probability that he makes four deals after speaking with eight customers in a week?