How many ways are there to choose a dozen donuts from 20 varieties
a) If there are no two donuts of the same variety?
b) If all donuts are of the same variety?
c) If there are no restrictions?
d) If there are at least two varieties among the dozen donuts chosen?
e) If there must be at least six blueberry-filled donuts?
f) If there can be no more than six blueberry-filled donuts?