What is the relation between n ( A ) , n ( B ) , n ( A ∩ B ) and n ( A ∪ B ) ?

Here, n ( A ) = the number of elements in set A . n B = the number of elements in set B . n A ∩ B = the number of elements in set A intersection B meaning the number of elements which are common in set A & B . n A U B = the number of elements combined together in set A or B or both. Thus, the relation between n ( A ) , n ( B ) , n ( A ∩ B ) and n ( A ∪ B ) is given by n A U B = n A + n B - n A ∩ B . This is the required answer.