Question

Let n(A u B)=50 , n(B)=20, n(A)=45, and U=A u B.

Answer 1

you meant P(A∩B)?
n(A U B) = n(A) + n(B) - n(A∩B)
50 = 45 + 20 - n(A∩B)

you can do the math

Answer 2

what's the question here ?

Answer 3

Find (A` u B`)

Answer 4

(A' U B') = (A∩B)' = U - (A∩B)
50 - 15 = 35