Finding P(B) given P(A) and P(A or B)

[daewoo]

Homework Statement

Suppose that P(A) = 0.8 and P(A or B) = 0.9 , determine P(B) If
a) A and B are independent
b) A and B are mutually exclusive

Homework Equations

for Independence P(A and B) = P(A) x P(B)
for Mutually Exclusive P(A or B) = P(A) + P(B)

The Attempt at a Solution

for A) just by the formula i can say that P(A and B) = P(A) x P(B)
but i don't see still how i can find P(B), the P(A or B) part is really confusing to me.

 

[HallsofIvy]

For not mutually exclusive, part (A), p(A or B)= P(A)+ P(B)- P(A and B). Put the values into both that AND P(A and B)= P(A)xP(B).

 

[daewoo]

hm thanks, but how do you find out what P(B) alone is because i can't use that formula anyways.

 

[HallsofIvy]

You can't use WHAT formula? And why not?