1.0.2x0.2x0.8=2.0.2x0.6+0.8x0.4=3.0.8x0.8x(0.4x0.6+0.4x0.6）+0.8x0.2x2x0.4x0.4= 答案自己算
（1-0.8）*（1-0.8）*0.8=0.032
