If A and B are two sets such that A has 40 elements, A ∪ B has 60 elements and A ∪ B has 10 elements, how many elements does B have?

Given in the question ,
n(A) = 40, n(A ∩ B) = 60 and n(A ∩ B) = 10.
As we know the formula,
n(A ∪ B) = n(A) + n(B) – n(A ∩ B)
Putting these values in the formula, we get
60 = 40 + n(B) − 10
⇒ n(B) = 30.