Question

Could somebody explain this to me?

List all the subsets of set:
{s, t}

Answer 1

A subset is a set which does not contain any element that is not in the original set. One such set would be {s} because s is in the original set {s, t}. Similarly, {t} is a subset of {s, t}. Also, if you read my definition carefully, you can check that {}, which does not contain any element, is a subset of {s, t}. Finally, {s, t} is a subset of {s, t} because every element of the first set is in the second set.

Answer 2

A systematic way to find all subsets is the following: (I think you'll get the idea)