If A and B are two sets such than n(A) = 54, n(B) = 39 and n(B – A) = 13 then find n(A ∪ B). Hint n(B) = n(B – A) + n(A ∩ B) ⇒ n(A ∩ B) = (39 – 13) = 26.

Given: n(A) = 54, n(B) = 39, n(B – A) = 13
Using the hint
n(B) = n(B – A) + n(A ∩ B)
⇒ 39 = 13 + n(A ∩ B)
⇒ n(A ∩ B) = 39 – 13
⇒ n(A ∩ B) = 26
Visualizing the hint given,