Number of elements in each of universal set, set A and set B are represented as n(U), n(A), n(B) respectively. The number of elements that are not in set A and not in set B is given by
n(U) - n(A U B)
No of elements not in set A and not in set B is represented in set notation as AI ∩ BI.
So, n(AI ∩ BI) = n(A U B)I
= n(U) - n(A U B)