Each of the `n` urns contains 4 white and 6 black balls. The `(n+1)` th urn contains 5 white and 5 black balls. One of the `n+1` urns is chosen at random and two balls are drawn from it without replacement. Both the balls turn out to be black. If the probability that the `(n+1)` th urn was chosen to draw the balls is 1/16, then find the value of `n` .