Find the smallest set A such that A ∪
{1,2} = {1,2,3,5,9}
The smallest set A such that
A ∪ {1,2} = {1,2,3,5,9} is {3.5.9}
∴ {3,5,9} ∪ {1,2} = {1,2,3,5,9}
Any other set B such that B ∪ {1,2} = {1,2,3,5,9} will contain A. For example we can take B to be {1,3,5,9} or {1,2,3,5,9} Clearly B contains A = {3,5,9}.