4. The probability function for the number of insurance policies John will sell to a customer is given by f(X) = .5 - (X/6) for X = 0, 1, or 2 a. Is this a valid probability function? Explain your answer. b. What is the probability that John will sell exactly 2 policies to a customer? c. What is the probability that John will sell at least 2 policies to a customer? d. What is the expected number of policies John will sell? e. What is the variance of the number of policies John will sell?