1)3*(1/4)*(3/4)*(3/4)=3/64
2)中三次的概率=(3/4)*(3/4)*2*(1/5)*(4/5)+2*(1/4)*(3/4)*(4/5)*(4/5)=21/50
中四次的概率=(3/4)*(3/4)*(4/5)*(4/5)=18/50
21/50+18/50=39/50
1)3*(1/4)*(3/4)*(3/4)=3/64
2)中三次的概率=(3/4)*(3/4)*2*(1/5)*(4/5)+2*(1/4)*(3/4)*(4/5)*(4/5)=21/50
中四次的概率=(3/4)*(3/4)*(4/5)*(4/5)=18/50
21/50+18/50=39/50