Question

把两封信随机投入A,B,C三个空信箱,则信箱A的信件数X的均值EX是?

Answer 1

A中有俩封信概率为(1/3)*(1/3)=1/9 有一封信概率为2*(1/3)*(2/3)=4/9

EX=2*(1/9)+1*(4/9)=2/3