In a 10 years period the number of earthquakes registering at least 2.5 on the Richter Scale and having an epicenter within 40 miles of a city center follows a Poisson distribution with mean 20.  Use the normal approximation to the Poisson distribution to compute  the probability that at least 23 such earthquakes will strike next decade. This probability is: 

 (Hint: You should correct for continuity).