請教GWD25-11

oneseven

Q11

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

 

A.     24

B.      36

C.     72

D.     144

E.      216

看過总汇裡面寫 9^3-P(3,9)-9

但我還是不明白算式的意思

麻煩牛牛解釋一下