disjoint subsets

Question

a and b are disjoint subsets except assume n(u)=70, n(a)=35, and n(B)=15.find n((AuB)')

Answer 1

n(AUB) = n(A) + n(B) - n(A∩B)

 

Since the sets A and B are disjoint, n(A∩B) = 0.  Hence:

 

n(AUB) = n(A) + n(B) = 35 + 15 = 50

 

n((AUB)') = n(u) - n(AUB) = 70 - 50 = 20