A man buys six donuts, each of which is plain donut, a powered donut or glazed donut. How many possible selections are there if he buys at least one donut of each kind? Please describe work
A. Answer the question above by listing all possible selections in a table
B. Answer the question above by determining the coefficient of x^6 in a product of polynomials and/or power series
C. Answer the question above by computing (s+t-1)/s for an appropriate choice of s and t