If n(A) = 20, n(B) = 30 and n(A ∪ B) = 40, find n(A ∩ B).
Answer: B
We use the formula for the cardinality of the union of two sets:
n(A ∪ B) = n(A) + n(B) - n(A ∩ B)
Substituting the given values:
40 = 20 + 30 - n(A ∩ B)
40 = 50 - n(A ∩ B)
n(A ∩ B) = 50 - 40 = 10