There are `N + 1` identical boxes each containing N wall clocks. `r^(th)` box contains `(r - 1)` defective and `(N-r+1)` effective clocks for `1lerleN+1.`A box is selected at random and from this box a clock is chosen at random and is found to be effective, the probability that it is from km box is `(2N – 2 k +lambda)/(N^2+N)` then find `lambda.`