The following table gives the probabilities gives the number of police officers who will call in sick (daily) for a large police force. Number of police officers,
X 4 5 6 7 8 9 10
Probability, P(X) 0.08 0.21 ? 0.19 0.16 0.22 0.03
a) Find the mean of this distribution.
b) Find variance and standard deviation.