#### Answer

n(A) + n(B) - n(A∩B)

#### Work Step by Step

The formula for the cardinal number of elements in set A or set B is n (A∪B) = n(A) + n(B) - n(A∩B).
To find the number of elements in the union of finite sets A and B, add the number of elements in A and the number of elements in B. Then subtract the number of elements common to both sets. We perform this subtraction so that we do not count the number of elements in the intersection twice, once for n(A), and again for n(B).