The number of imperfections in an object follows a Poisson distribution with a mean=8.3. If the number of imperfections is 4 or fewer, the object is called "top quality." If the number of imperfections is 5 or more, the object is called "normal quality." The number of imperfections in different objects are independent of each other.
a. A set of seven articles is taken. What is the probability that the set has exactly two top-quality objects?
b. A set of eight articles is taken. What is the expectation and standard deviation of the number of normal-quality objects in the set?