Subsets

Let A = {1, 2, 3, 4, 5}. As S runs through the 2element subsets of A, A \ S runs through the 3element subsets of A.

S  A \ S
{1, 2}  {3, 4, 5}
{1, 3}  {2, 4, 5}
{1, 4}  {2, 3, 5}
{1, 5}  {2, 3, 4}
{2, 3}  {1, 4, 5}
{2, 4}  {1, 3, 5}
{2, 5}  {1, 3, 4}
{3, 4}  {1, 2, 5}
{3, 5}  {1, 2, 4}
{4, 5}  {1, 2, 3}