If the probability of A given B is 0.8 and the probability of A is 0.12 what is the probability of B

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.