You have 25 cards, 15 distinguishable envelopes (i.e. envelopes are labeled 1,2,3...,15). You may put any non-negative number of cards into an envelope. In how many ways can you put the 25 cards if

a) the cards are distinguishable (e.g., if each has different message on it)

b) cards are identical

c) cards are identical and no card can be left empty

I can't figure out the answers for this problem. Help is much appreciated!