Question

将4封不同的信随机投入3个不同信箱,则3个信箱都不空的概率是

Answer 1

3个信箱都不空的概率是 C(4,2)*A(3,3)/3^4=6*6/81=4/9

---------------------不要用排列组合做:

1-3*(1/3)^4-3*((2/3)^4-2*(1/3)^4)

=1-1/27-3*(16/81-2/81)

=1-1/27-14/27

=12/27

=4/9