19. (5%) A traffic light can be in one of the three states: Green
(G), Red (R), and Yellow (Y). The light changes in a random fashion. At any one time, the light can be in only one state. The experiment consists of observing the state of the light. Let a random variable X(') be defined as follows: X
(G)=-1, X(R)=0, X(Y) = π； assume that P
(G) = P(Y) = 0.5 P(R), what is P(X ≦ 3)?
(A)1/4
(B) 1/2
(C) 3/4
(D)1
(E) 0
答案：99
統計：A:0,B:0,C:0,D:0,E:0
難度：計算中