15. Let X and Y be two discrete random variables with the identical set of possible values A = {a,

b, c}, where

a, b, and c are three different real numbers. Show that if E(X) =

E(Y ) and Var(X) =Var(Y ), then X and Y are identically distributed. That is, P(X = t) = P(Y = t) for t =

a, b, c.