Um $A$ has 5 white and 7 black balls. $\operatorname{Urn} B$ has 3 white and 12 black balls. We flip a fair coin. If the outcome is heads, then a ball from urn $A$ is selected, whereas if the outcome is tails, then a ball from urn $B$ is selected. Suppose that a white ball is selected. What is the probability that the coin landed tails?