Two straight roads diverge from an intersection at an angle of $60\backslash$deg $.$ Two cars leave the intersection at the same time, the first traveling down one road at $38$ mi$/$h and the second traveling down the other road at $64$ mi$/$h$.$ How fast $($in mi$/$h$)$ is the distance between the cars changing after half an hour? $[$Hint: Use the Law of Cosines.$]($Round your answer to one decimal place.$)$