Ok, so consider the following, the items are labeled (1) to (31) and each is either on the list or it is not. So we have 2 different states for each of these items. Now the question is, how many possible combinations exist.
Hint 1: >!If there exist x possibilities for item (1) and y possibilities for item (2), then there exists x\*y possibilities for the list (1)(2).!<
Hint 2: >!As a\*a\*a\*a = a\^4!<
Solution: >!2\^31 though you might want to subtract 1 if the possibility of an empty list does not exist.!<