GWD25-11

of the three digits positive integers that have no digits equal to zero, how many have two digits that are equal to each other and remaining digits different from above two?

the answer is: 216

any NN tell me how to derive this answer, thanks!!! 

三位数里，其中两位相同，另一位不同的有几个。

P(3,3)/P(2,2) * C(1,9)*C(1,8) = 3*72 = 216

感谢!

能说一下思路吗?P(3,3)/P(2,2)代表什末?我的数学挺差的.......