Patel's weekly salary is n rupees. Each week he uses 4n/5 rupees for expenses and saves the rest. At those rates, how many weeks will it take Patel to save 5000 rupees, in terms of n?

Eight points are equally spaced on a circle. If 4 of the 8 points are to be chosen at random, what is the probability that a quadrilateral having the 4 points chosen as vertices will be square?