Let Xn = {1,2,3,.n) and let a subset A of Xn be chosen so that every pair of elements of A differ by at least 3. (For example, if n = 5, A can be ϕ, {2} or {1,5} among others). When n = 10, let the probability that 1∈A be p and let the probability that 2∈A be q. Then