We can't tell from the information given. What we do know is
P(A ∩ B) = P(A|B) P(B)
We're given the conditional probability but not the probability of the conjunction.
If we were told P(A|B)=0.8 and P(A ∩ B)=0.12 we could conclude
P(B)=P(A ∩ B)/P(A|B) = 0.12/0.8 = 0.15
but that wasn't the problem asked.
