Please think about this problem for ten minutes and then repost it in the Discrete Math subforum. I am asking you to think because it seems that you copy-pasted the problem from a PDF file into the forum without even thinking if what you posted makes sense. For example, in the phrase "Let A be the family of all subsets A," the two A's must be typeset in different fonts. And what about the essential part (a), which you omitted? Finally, please describe your difficulty in solving the problem.