A lady invites 12 people for dinner at her house. Unfortunately the dining table can only seat 6 people. Her plan is that if 6 or fewer guests come, then they will be seated at the table (i.e., they will have a sit-down dinner); otherwise she will set up a buffet-style meal. The probability that each invited guest will come to dinner is 0.4, and each guest's decision is independent of other guests' decisions. Determine the following:
a. The probability that she has a sit-down dinner
b. The probability that she has a buffet-style dinner
c. The probability that there are at most three guests