答对0个是:C50(其中5是下标,1是上标,下同）*(3/4)*(3/4)*(3/4)*(3/4)*(3/4)=243/1024
答对1个是:C51*(1/4)*(3/4)*(3/4)*(3/4)*(3/4)=405/1024
答对2个是:C52*(1/4)*(1/4)*(3/4)*(3/4)*(3/4)=270/1024
答对3个是:C53*(1/4)*(1/4)*(1/4)*(3/4)*(3/4)=90/1024
答对4个是:C54*(1/4)*(1/4)*(1/4)*(1/4)*(3/4)=15/1024
答对5个是:C55*(1/4)*(1/4)*(1/4)*(1/4)*(1/4)=1/1024
至少答对2个题：即为至多答对一个的反面,即1-答对1个的概率-答对0个的概率=1-243/1024-405/1024=376/1024=47/128