Question

Can you list all subsets of the set (3,6,9,12) ?

Answer 1

{}

{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 $2^n$ 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})