Over half of all earthquakes in the U. S. occur in Alaska. Earthquakes with a magnitude greater than 8 on the Richter scale occur in Alaska on average every 13 years. Suppose that these occur according to a Poisson. Determine the probability that (a) no such earthquake occurs in a given decade, (b) two consecutive earthquakes of such magnitude are at least 7 years apart, (c) a 15-year period has exactly one such earthquake, and (d) two consecutive decades have exactly one such earthquake each.