Problem #1

If P(a) = .4 , P(b)= .5 and P(a ∩ b) = .3 find the following

a). P (a U b) = .6 because P (a U b) = P(a) + P(b) - P(a ∩ b)

b.) P(a' U b') = ?

c.) P(a U b') = .3 because P (a U b) = P(a ∩ b') + P(a ∩ b)

It's b.) that I'm not sure of since they're both compliments and I don't know what formula to use

Also Problem #2 (separate probabilities from #1)

If S = a U b , P(a) = .7, P(b) = .9, find

P(a ∩ b) = ?

and

P( (a ∩ b)' ) = ?

For these two problems I'm not really sure where to begin. Would S simply be 1 since P(S) is all elements of the sets and and b?