There are two boxes I and II. Box I contains 3 red and 6 black balls. Box II contains 5 red and ' $n$ ' black balls. One of the two boxes, box I and box II is selected at random and a ball is drawn at random. The ball drawn is found to be red. If the probability that this red ball comes out from box II is $\;\frac{3}{5}$, find the value of ' $n$ '.