1-0.4*0.2-0.6*0.8=0.6*0.2+0.4*0.8=0.44就是1-p（都不命中）-p（都命中）或者p（甲命中乙没命中）+p（乙命中甲没命中）
13/50
(1/5)(2/3)(3/4)+(4/5)(1/3)(3/4)+(4/5)(2/3)(1/4)=13/30
