{}
{3}, {6}, {9}, {12}
{3,6}, {3,9}, {3,12}
{6,9}, {6,12}
{9,12}
{3,6,9}, {3,6,12}
{3,9,12}
{6,9,12}
{3,6,9,12}
A handy way to check if you've got them all is the number of subsets equals [tex]2^n[/tex] where n is the number of elements in the set. (For instance, I could only find 15 to start with but knew there was another, it was that hidden {6,9,12})
