# Easy Probability Question

• Nov 22nd 2010, 01:56 PM
Heathcliff
Easy Probability Question
If P(A) = 0.2, P(A|B) = 0.3, P(A u B) = 0.4. Find P(B).

Reminding you that:
P(A n B) = P(A) + P(B) - P(A u B)
P(A u B) = P(A) + P(B) - P(A n B)

& that P(A|B) = P(A n B) / P(B)

I think I've got to use simultaneous equations... I just can't figure out where.
Use this fact: $P(B\cap A)=0.3P(B)$.
& $0.4=0.2+P(B)-P(B\cap A)$