不迟到的概率：
没有红灯时：
(1-1/2)x(1-1/2)x(1-1/3)x(1-1/3)=4/36
一次红灯时：
1/2x(1-1/2)x(1-1/3)x(1-1/3)x2+(1-1/2)x(1-1/2)x1/3x(1-1/3)x2=12/36
两次红灯时：
1/2x1/2x(1-1/3)x(1-1/3)+1/2x(1-1/2)x1/3x(1-1/3)x4+(1-1/2)x(1-1/2)x1/3x1/3=13/36
P=29/36
E(x) x=0,1,2,3,4
P(x=0)=(1-1/2)x(1-1/2)x(1-1/3)x(1-1/3)=4/36
P(x=1)=1/2x(1-1/2)x(1-1/3)x(1-1/3)x2+(1-1/2)x(1-1/2)x1/3x(1-1/3)x2=12/36
P(x=2)=1/2x1/2x(1-1/3)x(1-1/3)+1/2x(1-1/2)x1/3x(1-1/3)x4+(1-1/2)x(1-1/2)x1/3x1/3=13/36
P(x=3)=1/2x1/2x1/3x(1-1/3)x2+(1-1/2)x1/2x1/3x1/3x2=6/36
P(x=4)=1/2x1/2x1/3x1/3=1/36
E(x)=0x4/36+1x12/36+2x13/36+3x6/36+4x1/36=3/5
自己做的