Customers arrive at the checkout counter of a supermarket at an average rate of 10 per hour, and these arrivals follow a Poisson distribution. Using each of the following two methods, find the probability that exactly 4 customers will arrive at this checkout counter during a 2-hour period.
a. Use the arrivals in each of the two no overlapping 1-hour periods and then add these.
b. Use the arrivals in a single 2-hour period.