Answer
$n(A \bigcup B) = n(A) + n(B) - n(A\cap B)$
Work Step by Step
We can count the number of elements of the union of two sets by adding the number of elements each set has and subtracting the number of elements of their intersection. Therefore: $n(A \bigcup B) = n(A) + n(B) - n(A\cap B)$
