試題詳解

311. Employees of a large company all choose one of three levels of health insurance coverage, for which premiums, denoted by X , are 1, 2, and 3, respectively. Premiums are subject to a discount, denoted by Y, of 0 for smokers and 1 for non-smokers. The joint probability function of X and Y is given by

  Calculate the variance of X Y− , the total premium paid by a randomly chosen employee.
(A) 0.20
(B) 0.69
(C) 0.74
(D) 1.90
(E) 2.65